Thermally strengthened consumer electronic glass and related systems and methods

ABSTRACT

A strengthened cover glass or glass-ceramic sheet or article as well as processes and systems for making the strengthened glass or glass-ceramic sheet or article is provided for use in consumer electronic devices. The process comprises cooling the cover glass sheet by non-contact thermal conduction for sufficiently long to fix a surface compression and central tension of the sheet. The process results in thermally strengthened cover glass sheets for use in or on consumer electronic products.

RELATED SYSTEMS AND METHODS

This application is a continuation of U.S. application Ser. No.15/749,015 filed Jan. 30, 2018, which is a U.S. national-stage entry ofInternational Application No. PCT/US16/44406 filed Jul. 28, 2016, whichclaims the benefit of priority of U.S. Application Nos. 62/228,851 filedJan. 29, 2016, 62/236,296 filed Oct. 2, 2015, and 62/286,192 filed Jan.22, 2016 and also claims the priority benefit as continuation-in-part ofU.S. application Ser. No. 14/814,232 filed Jul. 30, 2015, which issuedMar. 29, 2016 as U.S. Pat. No. 9,296,638, Ser. No. 14/814,274 filed Jul.30, 2015, which issued as U.S. Pat. No. 9,776,905 on Mar. 10, 2017, Ser.No. 14/814,293 filed Jul. 30, 2015, which issued as U.S. Pat. No.9,802,853 on Feb. 4, 2016, Ser. No. 14/814,303 filed Jul. 30, 2015,which issued as U.S. Pat. No. 9,783,448 on Oct. 10, 2017, Ser. No.14/814,363 filed Jul. 30, 2015, which issued as U.S. patent Ser. No.10/005,691 on Jun. 26, 2018, Ser. No. 14/814,319 filed Jul. 30, 2015,which issued as U.S. Pat. No. 9,975,801 on May 22, 2018, and Ser. No.14/814,335 filed Jul. 30, 2015, which issued as U.S. patent Ser. No.10/077,204 on Sep. 18, 2018, each of which is hereby incorporated byreference herein in its entirety.

BACKGROUND

The disclosure relates generally to thermally conditioned (e.g.,strengthened, tempered, heated, etc.) cover glass for consumerelectronic applications, and specifically relates to thermallystrengthened glass and to related methods and systems for the thermalstrengthening of glass for consumer electronic applications,particularly for thin consumer electronic glass sheets (also called“cover glass” herein).

In thermal (or “physical”) strengthening of glass sheets, a glass sheetis heated to an elevated temperature above the glass transitiontemperature of the glass and then the surfaces of the sheet are rapidlycooled (“quenched”) while the inner regions of the sheet cool at aslower rate. The inner regions cool more slowly because they areinsulated by the thickness and the fairly low thermal conductivity ofthe glass. The differential cooling produces a residual compressivestress in the glass surface regions, balanced by a residual tensilestress in the central regions of the glass.

Thermal strengthening of glass is distinguished from chemicalstrengthening of glass, in which surface compressive stresses aregenerated by changing the chemical composition of the glass in regionsnear the surface by a process such as ion diffusion. In some iondiffusion based processes, exterior portions of glass may bestrengthened by exchanging larger ions for smaller ions near the glasssurface to impart a compressive stress (also called negative tensilestress) on or near the surface. The compressive stress is believed tolimit crack initiation and/or propagation.

Thermal strengthening of glass also is distinguished from glassstrengthened by processes in which exterior portions of the glass arestrengthened or arranged by combining two types of glass. In suchprocesses, layers of glass compositions that have differing coefficientsof thermal expansion are combined or laminated together while hot. Forexample, by sandwiching molten glass with a higher coefficient ofthermal expansion (CTE) between layers of molten glass with a lower CTE,positive tension in the interior glass compresses the outer layers whenthe glasses cool, again forming compressive stress on the surface tobalance the positive tensile stress. This surface compressive stressprovides strengthening.

Thermally strengthened consumer electronic glass or cover glass hasadvantages relative to unstrengthened glass. The surface compression ofthe strengthened consumer electronic glass or cover glass providesgreater resistance to fracture than unstrengthened glass. The increasein strength generally is proportional to the amount of surfacecompression stress. If a sheet possesses a sufficient level of thermalstrengthening, relative to its thickness, then if the sheet is broken,generally it will divide into small fragments rather than into large orelongated fragments with sharp edges. Glass that breaks intosufficiently small fragments, or “dices,” as defined by variousestablished standards, may be known as safety glass, or “fully tempered”glass, or sometimes simply “tempered” glass.

Because the degree of strengthening depends on the temperaturedifference between the surface and center of the glass sheet duringquenching, thinner glasses require higher cooling rates to achieve agiven stress. Also, thinner glass generally requires higher values ofsurface compressive stress and central tension stress to achieve dicinginto small particles upon breaking. Accordingly, achieving desirablelevels of tempering in glass with thicknesses of around 3 mm or less hasbeen exceedingly challenging, if not impossible.

Aspects of the present disclosure also relate to consumer electronicglass or cover glass that has a stress profile for strengtheningexterior portions thereof. Consumer electronic glass, such as coverglass for use on any surface of a consumer electronic device, may beused for a broad range of applications Such applications include cellphones, tablets, mobile phones, personal computers, notebook computers,digital signage, digital white board, etc.

Aspects of the present disclosure also relate generally to glass orglass-ceramic that has a stress profile for strengthening exteriorportions thereof. Glass and glass-ceramic articles, such as sheets ofglass, may be used for a broad range of applications. Examples of suchapplications include use in windows, countertops, containers (e.g.,food, chemical), use as a backplane, frontplane, cover glass, etc., fora display device (e.g., tablet, cellular phone, television), use as ahigh-temperature substrate or support structure, or other applications.

SUMMARY

This disclosure relates, in part, to highly strengthened thin consumerelectronic glass or cover glass sheets and articles, and to methods,processes, and systems that achieve surprisingly high levels of heatstrengthening of consumer electronic glass or cover glass sheets atthicknesses not achieved in the past. In various embodiments, theprocess and method of the current disclosure is believed to surpass theconsumer electronic glass or cover glass thickness limits and heattransfer rates provided by conventional convective gas thermalstrengthening processes without the need to contact the consumerelectronic glass or cover glass with liquid or solid heat sinks. In suchsystems and processes, during quenching, the cover glass is contactedonly with a gas. The systems and methods disclosed enable thermalstrengthening, including up to “full temper” or dicing behavior, incover glass sheets having thicknesses down to at least as thin as 0.1 mm(in at least some contemplated embodiments); and in some embodiments,provides this strengthening in a thin cover glass sheet that also has alow roughness and a high degree of flatness resulting from the lack ofliquid or solid contact during quenching. In various embodiments, theseadvantageous cover glass sheet material properties are provided by asystem and method with substantially lower quenching power requirements,as compared to conventional convective glass tempering systems.

One embodiment of the disclosure relates to a process for thermallystrengthening a consumer electronic glass or cover glass material. Theprocess includes providing article formed from a glass material. Theprocess includes heating the article above a glass transitiontemperature of the cover glass material. The process includes moving theheated article into a cooling station. The cooling station includes aheat sink having a heat sink surface facing the heated article and a gasgap separating the heat sink surface from the heated article such thatthe heat sink surface does not touch the heated article. The processincludes cooling the heated article to a temperature below the coverglass transition temperature such that surface compressive stresses andcentral tensile stresses are created within the article. The article iscooled by transferring thermal energy from the heated article to theheat sink by conduction across the gap such that more than 20% of thethermal energy leaving the heated article crosses the gap and isreceived by the heat sink.

Another embodiment of the disclosure relates to a system for thermallystrengthening a cover glass sheet. The system includes a heating stationincluding a heating element delivering heat to the cover glass sheet,and the cover glass sheet includes a first major surface, a second majorsurface and a thickness between the first and second major surfaces. Thesystem includes a cooling station, including opposing first and secondheat sink surfaces defining a channel therebetween such that duringcooling the cover glass sheet is located within the channel. The systemincludes a gas bearing delivering pressurized gas to the channel suchthat the cover glass sheet is supported within the channel withouttouching the first and second heat sink surfaces, and the gas bearingdefines a gap area. The gas bearing delivers a gas into the channel suchthat a total mass flow rate of gas into the channel is greater than zeroand less than 2 k/gC_(p) per square meter of gap area, where k is thethermal conductivity of a gas within the channel evaluated in thedirection of heat conduction, g is the distance between the cover glasssheet and the heat sink surface, and C_(p) is the specific heat capacityof the gas within the channel.

Another embodiment of the disclosure relates to a strengthened consumerelectronic glass/cover glass or glass-ceramic article. The articleincludes a first major surface, a second major surface opposite thefirst major surface and an interior region located between the first andsecond major surfaces. The article includes an average thickness betweenthe first major surface and second major surface of less than 2 mm. Thearticle includes at least 70% silicon dioxide by weight. An ion contentand chemical constituency of at least a portion of both the first majorsurface and the second major surface is the same as an ion content andchemical constituency of at least a portion of the interior region. Thefirst major surface and the second major surfaces are under compressivestress and the interior region is under tensile stress, and thecompressive stress is greater than 150 MPa. A surface roughness of thefirst major surface is between 0.2 and 2.0 nm R_(a) roughness.

Another embodiment of the disclosure relates to a strengthened consumerelectronic glass or cover glass on a consumer electronic product. Inembodiments, the consumer electronic product includes an electronicdisplay with a front surface, a back surface, and at least one sidesurface. In embodiments, a glass-based layer or cover glass is providedat least partially over the electronic display. In embodiments, theglass-based layer or cover glass includes a first major surface oppositea second major surface with an interior region located therebetween. Inembodiments, the glass-based layer or cover glass includes an averagethickness between the first major surface and second major surface ofless than 2 mm. In embodiments, an ion content and chemical constituencyof at least a portion of both the first major surface and the secondmajor surface is the same as an ion content and chemical constituency ofat least a portion of the interior region. In embodiments, the firstmajor surface and the second major surfaces are under compressive stressand the interior region is under tensile stress, and the compressivestress is greater than 150 MPa. In embodiments, a surface roughness ofthe first major surface is between 0.2 and 2.0 nm R_(a) roughness.

Another embodiment of the disclosure relates to a strengthened consumerelectronic glass or cover glass for a consumer electronic product. Inembodiments, the consumer electronic product includes a housing with afront surface, a back surface, and at least one side surface. Inembodiments, electrical components, including at least a controller, amemory, and a display, are provided at least partially internal to thehousing. In embodiments, a glass-based layer or back glass is providedon or over the back surface of the housing. In embodiments, theglass-based layer or back glass includes a first major surface oppositea second major surface with an interior region located therebetween. Inembodiments, the glass-based layer or back glass includes an averagethickness between the first major surface and second major surface ofless than 2 mm. In embodiments, an ion content and chemical constituencyof at least a portion of both the first major surface and the secondmajor surface is the same as an ion content and chemical constituency ofat least a portion of the interior region. In embodiments, the firstmajor surface and the second major surfaces are under compressive stressand the interior region is under tensile stress, and the compressivestress is greater than 150 MPa. In embodiments, a surface roughness ofthe first major surface is between 0.2 and 2.0 nm R_(a) roughness.

Another embodiment of the present disclosure relates to a consumerelectronic product including a strengthened consumer electronic glass orcover glass. In embodiments, the consumer electronic product includes ahousing with a front surface, a back surface, and at least one sidesurface. In embodiments, electrical components are provided at leastpartially internal to the housing. In embodiments, the electricalcomponents include at least a display. In embodiments, the consumerelectronic glass or cover glass is provided as or adjacent the frontsurface of the housing. In embodiments, the glass based layer or coverglass includes a first major surface and a second major surfaceseparated by the thickness. In embodiments, the first major surface ofthe glass-based layer or cover glass is flat to 100 μm total indicatorrun-out (TIR) along any 50 mm or less profile of the first major surfaceof the glass-based layer or cover glass. In embodiments, the glass-basedlayer or cover glass includes a glass material having a low temperaturelinear CTE, expressed in 1/° C., of α^(S) _(CTE), a high temperaturelinear CTE, expressed in 1/° C., of α^(L) _(CTE), an elastic modulus,expressed in GPa, of E, a strain temperature, expressed in units of °C., of T_(strain), and a softening temperature, expressed in units of °C., of T_(soft). In further embodiments, the first major surface of thesecond glass-based layer has a thermally induced surface compressivestress of less than 600 MPa and greater than

${\frac{{P_{1}(h)}*t}{\left( {{P_{2}(h)} + t} \right)} \cdot E \cdot \left\lbrack {{T_{strain} \cdot \alpha_{CTE}^{s}} + {\alpha_{CTE}^{L} \cdot \left( {T_{soft} - T_{strain}} \right)}} \right\rbrack};$in units of MPa;wherein P₁ is given by

${{91{0.2}} - {259.2 \cdot {\exp\left( {- \frac{h}{{0.1}43}} \right)}}};$P₂ is given by

${{{2.5}3} + \frac{2{3.6}5}{\left( {1 + \left( \frac{h}{{0.0}0738} \right)^{{1.5}8}} \right)}};$and h is greater than or equal to 0.020 cal/s·cm²·° C.

Yet another embodiment of the present disclosure relates to a consumerelectronic product including a strengthened consumer electronic glass orcover glass. In embodiments, the consumer electronic product includes ahousing with a front surface, a back surface, and at least one sidesurface. In embodiments, electrical components, including at least acontroller, a memory, and a display, are provided at least partiallyinternal to the housing. In embodiments, a glass-based layer or coverglass is provided at or adjacent the front surface of the housing. Inembodiments, the glass-based layer or cover glass is provided at leastpartially over the display. In embodiments, the glass-based layer orcover glass includes a first major surface opposite a second majorsurface with an interior region located therebetween. In embodiments,the first major surface is flat to 100 μm total indicator run-out (TIR)along any 50 mm or less profile of the first major surface. Inembodiments, the glass-based layer cover glass includes a glass materialhaving a softening temperature, expressed in units of ° C., of T_(soft)and an annealing temperature, expressed in units of ° C., of T_(anneal),and a surface fictive temperature measured on the first major surfacerepresented by Tfs, when expressed in units of ° C. In embodiments, theglass-based layer or cover glass having a non-dimensional surfacefictive temperature parameter θs given by(T_(fs)−T_(anneal))/(T_(soft)−T_(anneal)). In embodiments, the parameterθs is in the range of from 0.20 to 0.9.

Additional features and advantages will be set forth in the detaileddescription that follows, and, in part, will be readily apparent tothose skilled in the art from the description or recognized bypracticing the embodiments as described in the written description andclaims hereof, as well as the appended drawings.

It is to be understood that both the foregoing general description andthe following detailed description are merely exemplary, and areintended to provide an overview or framework to understand the natureand character of the claims.

The accompanying drawings are included to provide a furtherunderstanding and are incorporated in and constitute a part of thisspecification. The drawings illustrate one or more embodiments, andtogether with the description serve to explain principles and theoperation of the various embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 (Prior Art) is a graph of blower power required for “fulltempering” as a function of glass thickness.

FIG. 2 (Prior Art) is a graph of blower power required for “fulltempering” as a function of glass thickness for an old process ormachine O and a newer process or machine N.

FIG. 3 (Prior Art) is a graph of the old curve O and the new curve N ofFIG. 2 scaled to match and superimposed upon the graph of FIG. 1 .

FIG. 4 is a perspective view of a consumer electronic/cover glass orglass-ceramic article or sheet according to an exemplary embodiment.

FIG. 5 is a diagrammatic partial cross-section of a thermallystrengthened consumer electronic glass sheet or cover glass of FIG. 4according an exemplary embodiment.

FIG. 6 is a graphical representation of estimated tensile stress versusthickness for a glass or glass-ceramic article according to an exemplaryembodiment.

FIG. 7 shows a portion of a fractured cover glass or glass-ceramicarticle according to an exemplary embodiment.

FIG. 8 is a plot of fragmentation per square centimeter as a function ofpositive tensile stress from experiment.

FIG. 9 is a plot of the magnitude of negative tensile stress at thesurface as a function of initial hot zone temperature from experiment,showing a threshold to achieve dicing.

FIG. 10 is a plot of the non-dimensional surface fictive temperatureparameter θs for fictive temperatures obtained by one or moreembodiments of methods and systems of the present invention.

FIG. 11 is a plot of surface compression stresses calculated bysimulation for differing glass compositions, plotted against a proposedtemperability parameter Ψ for the various compositions shown.

FIGS. 12 and 13 are graphs of two parameters P₁ and P₂ as functions ofheat transfer coefficient h.

FIG. 14 is a graph of MPa of surface compression of a glass sheet as afunction of thickness t of the sheet in millimeters, showing regions ofperformance newly opened by one or more embodiments of the systems andmethods of the present disclosure.

FIG. 15 is a graph showing compressive stress as a function of thicknessplotted for selected exemplary embodiments of tempered glass sheets ofthe present disclosure.

FIG. 16 is a flow chart illustrating some aspects of a method accordingto the present disclosure.

FIG. 17 is a flow chart illustrating some aspects of another methodaccording to the present disclosure.

FIG. 18 is the graph of FIG. 3 with a region R and points A, B, A′ andB′ marked thereon to show a region in which the methods and systems ofthe present disclosure allow operation, in contrast to the prior art.

FIG. 19 is another representation of the region R and points A, B, A′and B′ of FIG. 18 , but shown adjacent to (and positioned relative tothe scale) of a reduced size copy of FIG. 2 .

FIG. 20 (Prior Art) is a graph of the required heat transfer coefficientneeded for tempering as a function of glass thickness.

FIG. 21 is a diagrammatic cross-section of a glass sheet being cooled byconduction more than by convection, according to an exemplaryembodiment.

FIG. 22 is a schematic cross-sectional diagram of a conductivestrengthening system according to an exemplary embodiment.

FIG. 23 is a perspective cut-away view of another embodiment of a systemsimilar to that of FIG. 22 according to an exemplary embodiment.

FIG. 24 is a perspective cut-away view of an alternative embodiment ofthe inset feature of FIG. 23 according to an exemplary embodiment.

FIG. 25 is a perspective cut-away view of yet another alternativeembodiment of the inset feature of FIG. 23 according to an exemplaryembodiment.

FIG. 26 is a flow chart illustrating some aspects of yet another methodaccording to an exemplary embodiment.

FIG. 27 is a perspective view of a building with glass windows accordingto an exemplary embodiment.

FIG. 28 is a perspective view of a display on a countertop according toan exemplary embodiment.

FIG. 29 is an exploded perspective view of a consumer electronic deviceor product including glass or glass-ceramic articles according to anexemplary embodiment.

FIG. 30 is a perspective view of a glass or glass-ceramic article orsheet according to an exemplary embodiment.

DETAILED DESCRIPTION

Applicant has recognized a need for improvements in thermal processingof cover glass, both in methods and systems for thermally strengtheningcover glass and the resulting thermally strengthened cover glass sheetsthemselves. For example, thinner, but strong optical-quality cover glasssheet materials and products comprising such cover glass sheets areuseful for a number of applications, including portable electronicdevices, consumer electronic products, cover glass, back glass, etc.Glass is very strong in compression but relatively weak against tensionat the surface. By providing compression at the surface of a sheet,balanced by tension at the center where there is no exposed surface, theuseful strength of a cover glass sheet is dramatically increased.However, while traditional thermal strengthening of cover glass isgenerally cheaper and faster relative to alternative methods ofstrengthening (e.g., chemical strengthening, lamination-basedstrengthening), traditional thermal strengthening of cover glass is notknown to be effective for strengthening thin cover glass (e.g., coverglass sheets of 2-3 mm or less). Traditional thermal cover glassstrengthening methods are typically thought to be limited to thickercover glass sheets because the level of strengthening depends on thetemperature difference created between the surface and the center of thecover glass sheet during quenching; and because of thermal conductionrate limitations of traditional strengthening methods, it is difficultto achieve significant temperature differences between the surface andthe center of a thin cover glass sheet due to the relatively evencooling that typically occurs throughout a thin glass sheet.

On the other hand, strengthening thin cover glass through ion exchangecan be time-consuming and cumbersome, such as requiring chemical bathingof the cover glass for extended periods of time. Laminating differenttypes of cover glasses directly to one another may require complicatedmanufacturing processes, such as involving a dual-isopipe fusion draw.

Therefore, a need exists for cover glass or glass-ceramic articleshaving stress profiles that result in strengthening of the cover glassfor a variety of uses such as in windows, countertops, devices, etc.made by processes that are less resource-intensive and/or cumbersomethan conventional processes. Specifically, processes and systemsdiscussed herein form cover glass articles having stress profiles thatstrengthen the exterior portions of the cover glass, which in turn actto mitigate cracking and damage while at the same time allowing for avariety of other desirable cover glass qualities (e.g., geometry,surface quality, transmittance of visible light, flexibility, etc.) tofacilitate the use in various cover glass or consumer electronic productapplications.

The present description provides improved methods and systems forutilizing thermal strengthening to produce highly strengthened coverglass materials, and in particular highly strengthened thin cover glasssheets. The methods and systems solve a variety of the limitations ofconventional cover glass strengthening processes, allowing for highlevels of strengthening in cover glass sheets with thicknesses less thanabout 3 mm, less than 2 mm, less than 1.5 mm, less than 1.0 mm, lessthan 0.5 mm, less than about 0.25 mm, and less than about 0.1 mm. Inparticular, Applicant has developed a system and method that provides avery high rate of thermal conduction forming a large enough temperaturedifferential between the surface and center of a cover glass sheet toprovide strengthening or tempering even in very thin cover glass sheets.

Overview of Conventional Thermal Tempering Technology and Limitations

Conventional industrial processes for thermally strengthening glassinvolve heating glass sheets in a radiant energy furnace or a convectionfurnace (or a “combined mode” furnace using both techniques) to apredetermined temperature, then gas cooling (“quenching”), typically viaconvection by blowing large amounts of ambient air against or along theglass surface. This gas cooling process is predominantly convective,whereby the heat transfer is by mass motion (collective movement) of thefluid, via diffusion and advection, as the gas carries heat away fromthe hot glass sheet.

In conventional tempering processes, certain factors can restrict theamount of strengthening typically consider possible in glass sheets,particularly thin glass sheets. Limitations exist, in part, because theamount of compressive stress on the finished sheet is related directlyto the size of the temperature differential between the surface and thecenter of the sheet, achieved during quenching. However, the larger thetemperature differential during quenching, the more likely glass is tobreak during quenching. Breakage can be reduced, for a given rate ofcooling, by starting the quench from a higher initial glass temperature.Also, higher starting temperatures typically allow the tempered glasssheet to achieve the full strengthening potential provided by highcooling rates. However, increasing the temperature of the sheet at thestart of the quench also has its own potential drawbacks. For example,high initial glass temperatures can lead to excessive deformation of thesheet as it becomes softer, again limiting the practically achievabletemperature differential.

In conventional tempering processes, sheet thickness also imposessignificant limits on the achievable temperature differential duringquenching. The thinner the sheet, the lower the temperature differentialbetween the surface and the center for a given cooling rate duringquenching. This is because there is less glass thickness to thermallyinsulate the center from the surface. Accordingly, thermal strengtheningof thin glass typically requires higher cooling rates (as compared tothermal strengthening of thicker glass) and, thus, faster removal ofheat from the external surfaces of the glass typically requiressignificant energy consumption in order to generate strengthening levelsof differential temperature between the inner and outer portions of theglass sheet.

By way of example, FIG. 1 shows the power required by air blowers (inkilowatts per square meter of glass sheet area) employed to blowsufficient ambient air to “fully temper” soda-lime glass (“SLG”), as afunction of glass thickness in millimeters, based on industry standardthermal strengthening processes developed 35 years ago. The powerrequired increases exponentially as the glass used gets thinner. Thus,glass sheets of about 3 mm in thickness were the thinnest fullythermally tempered commercial glass available for many years.

Further, the thinner the sheet, the greater the likelihood ofdeformation at a given softness (that is, at a given viscosity) of theglass. Therefore, decreasing thickness both reduces the achievabletemperature differential directly and, because of increased risk ofdeformation of the sheet, tends to reduce the opportunity to use highersheet temperatures to achieve the full benefits of higher cooling ratesand to prevent glass breakage caused by higher cooling rates. Thus, inconventional convective gas glass strengthening processes, higher ratesof cooling are achieved by increasing the rate of air flow, decreasingthe distance of air nozzle openings to the glass sheet surface,increasing the temperature of the glass (at the start of cooling), andoptionally, decreasing the temperature of the cooling air.

As a more recent example, the performance curves of FIG. 2 (Prior Art)were published using state of the art glass thermal strengtheningequipment. This improved equipment continues to use traditional airblown convective processes to cool the glass, but replaces rollers usedto support the glass during heating with a system that utilizes air tosupport the glass during at least the last stages of heating. Withoutroller contact, the glass can be heated to higher temperatures (andhigher softness/lower viscosity) prior to quenching, reportedly allowingthe production of fully tempered glass at 2 mm thickness. As shown inFIG. 2 , the reported blower power required to strengthen a 2 mm thicksheet is reduced from 1200 kW/m² to 400 kW/m² at the higher temperaturesenabled by using air to support the glass (curve N) as compared to usingrollers (curve O).

Although it represents progress to be able to produce fully tempered 2mm thick glass, scaling the old and new curves O and N of FIG. 2 tomatch the scale of FIG. 1 , as shown in FIG. 3 (Prior Art), shows thatthe improvement in performance achieved by the state of the artconvective tempering process (shown in FIG. 2 ) is relatively small andsimply an incremental change in the previous understanding of the energyneeds in convective strengthening of glass sheets. In FIG. 3 the old andnew curves O and N of FIG. 2 are scaled to match the graph of FIG. 1 ,and overlaid thereon (with the old curve O truncated at the top at 240kW/m² for easier viewing of the new curve N). From FIG. 3 it is apparentthat the technology represented by the curve N changes only slightly theperformance curve of convective gas quenching processes as glassthickness is decreased from 3 mm to 2 mm. The high operating point (400kW/m² of blower power for 2 mm glass) shows the extreme increase inpower still required to process thinner glass by this method. The sharpincrease in airflow and, thus, power needed suggests the difficulty, asa matter of both engineering practice and economics, in going below 2 mmthickness while producing fully tempered glass using conventionalconvective gas strengthening methods. Additionally, the very highairflows needed also could deform the shape of thinner sheets.Accordingly, to reach full temper of glass having a thickness of lessthan 2 mm or to reach full temper at 2 mm in glasses having coefficientsof thermal expansion (“CTE”) lower than that of soda-lime glasses usingthermal tempering, Applicant has identified that another temperingmethod/system is needed.

Alternative thermal strengthening methods to current commercialconvective gas strengthening have been tried as well, but each hascertain drawbacks relative to convective gas strengthening. Inparticular, typical alternative thermal strengthening methods thatachieve higher cooling rates generally require at least some liquid orsolid contact with the glass surfaces, rather than gas contact only.Such contact with the glass sheet can adversely affect glass surfacequality, glass flatness, and/or evenness of the strengthening process.These defects sometimes can be perceived by the human eye, particularlywhen viewed in reflected light. As described in more detail below, atleast in some embodiments, the conductive thermal tempering system ofthe present disclosure reduces or eliminates such contact-relateddefects.

Liquid contact strengthening, in the form of immersion in liquid bathsor flowing liquids, as well as in the form of spraying, has been used toachieve higher cooling rates than convective gas strengthening, but hasthe drawback of causing excessive thermal variations across a sheetduring the cooling process. In immersion or immersion-like spraying orflowing of liquids, large thermal variations over small areas can occurdue to convection currents that arise spontaneously within the liquidbath or liquid flow. In finer spraying, the discrete spray droplets andthe effects of nozzle spray patterns also produce significant thermalvariations. Excessive thermal variations tend to cause glass breakageduring thermal strengthening by liquid contact, which can be mitigatedby limiting the cooling rates, but limiting cooling rates also lowersthe resulting strengths that can be achieved. Further, the necessaryhandling of the sheet (to position or hold it within the liquid bath orliquid flow or liquid spray) also causes physical stress and excessivethermal variations from physical contact with the sheet, tending also tocause breakage during strengthening and limiting the cooling rates andresulting strengths. Finally, some liquid cooling methods, such as highcooling rate quenching by oil immersion and various spraying techniques,can alter the glass surface during such cooling, requiring later removalof glass material from the sheet surface to produce a satisfactoryfinish.

Solid contact thermal strengthening involves contacting the surface ofthe hot glass with a cooler solid surface. As with liquid contactstrengthening, excessive thermal variations, like those seen in liquidcontact strengthening, can easily arise during the quenching process.Any imperfection in the surface finish of the glass sheet, in thequenching surfaces, or in the consistency of the thickness of the sheet,results in imperfect contact over some area of the sheet, and thisimperfect contact may cause large thermal variations that tend to breakthe glass during processing and may also cause unwanted birefringence ifthe sheet survives. Additionally, contacting the hot glass sheet with asolid object can lead to the formation of surface defects, such aschips, checks, cracks, scratches, and the like. Achieving good physicalcontact over the entirety of the surfaces of a glass sheet also canbecome increasing difficult as the dimensions of the sheet increase.Physical contact with a solid surface also can mechanically stress thesheet during quenching, adding to the likelihood of breaking the sheetduring the process. Further, the extreme high rate temperature changesat the initiation of contact can cause breakage during sheet processingand, as such, contact cooling of thin glass substrates has not beencommercially viable.

Overview of Applicant's Thermally Strengthened Cover Glass and RelatedConductive Cooling Process and Method

The present disclosure surpasses the traditional processes describedabove to effectively, efficiently, and evenly thermally strengthen thincover glass sheets at commercial scales without generating various flawscommon in conventional processes, e.g., without damaging the surface ofthe cover glass, without inducing birefringence, without unevenstrengthening, and/or without causing unacceptable breakage, etc.Previously unobtainable, thin, thermally tempered/strengthened coverglass sheets can be produced by one or more of the embodiments disclosedherein. The systems and processes discussed herein accomplish this byproviding very high heat transfer rates in a precise manner, with goodphysical control and gentle handling of the cover glass. In particularembodiments, the processes and systems discussed herein utilize asmall-gap, gas bearing in the cooling/quenching section that Applicanthas identified as allowing for processing thin cover glass sheets athigher relative temperatures at the start of cooling, resulting inhigher thermal strengthening levels. As described below, this small-gap,gas bearing cooling/quenching section achieves very high heat transferrates via conductive heat transfer to heat sink(s) across the gap,rather than using high air flow based convective cooling. This high rateconductive heat transfer is achieved while not contacting the coverglass with liquid or solid material, by supporting the cover glass ongas bearings within the gap. As described below, Applicant has alsoidentified that, in at least some embodiments, the processes and systemsdiscussed herein form thermally strengthened cover glass, specificallythermally strengthened thin cover glass (e.g, for consumer electronicproducts), having one or more unique properties.

Some embodiments of cover glass sheets treated by methods and/or systemsaccording to the present disclosure have higher levels of permanentthermally induced stresses than previously known. Without wishing to bebound by theory, it is believed that the achieved levels of thermallyinduced stress are obtainable for a combination of reasons. The highuniformity of the heat transfer in the processes detailed herein reducesor removes physical and unwanted thermal stresses in the cover glass,allowing consumer electronic glass sheets to be tempered at higher heattransfer rates without breaking. Further, the present methods can beperformed at lower glass sheet viscosities (higher initial temperaturesat the start of quench), while still preserving the desired cover glassflatness and form, which provides a much greater change in temperaturein the cooling process, thus increasing the heat strengthening levelsachieved.

Thermally Tempered Cover Glass Sheet

As noted above, Applicant has developed a system and process for formingthermally strengthened cover glass sheets, particularly thin cover glasssheets, and as discussed in this section, the thermally strengthened,thin cover glass sheets formed as discussed herein have one or moreunique properties and/or combinations of properties, previouslyunachievable through conventional thermal or other tempering methods.The thermally tempered cover glass or consumer electronic glass of thepresent disclosure may be used in a variety of consumer electronicproducts (e.g., computers, tablets, personal handheld devices, touchsensitive displays, household appliances, mobile phones, portable mediaplayers, televisions, notebook computers, watches, tablet computers,etc.).

Thermally Tempered Cover Glass Sheet Structure and Dimensions

Referring to FIG. 4 and FIG. 5 , a thermally strengthened cover glasssheet having a high surface compressive stress and/or a high centraltension is shown according to an exemplary embodiment. FIG. 4 shows aperspective view of a thermally strengthened cover glass orglass-ceramic article or sheet 500, and FIG. 5 is a diagrammatic partialcross-section of thermally strengthened cover glass sheet 500 accordingto one or more embodiments.

As shown in FIG. 4 , a strengthened cover glass or glass-ceramic article500 (e.g., sheet, beam, plate), includes a first major surface 510, asecond major surface 520 (dotted line to back side of the sheet 500,which may be translucent as disclosed herein), and a body 522 extendingtherebetween. The second major surface 520 is on an opposite side of thebody 522 from the first major surface 510 such that a thickness t of thestrengthened cover glass or glass-ceramic sheet 500 is defined as adistance between the first and second major surfaces 510, 520, where thethickness t is also a dimension of depth. A width, w, of thestrengthened cover glass or glass-ceramic sheet 500 is defined as afirst dimension of one of the first or second major surfaces 510, 520orthogonal to the thickness t. A length, l, of the strengthened coverglass or glass-ceramic sheet 500 is defined as a second dimension of oneof the first or second major surfaces 510, 520 orthogonal to both thethickness t and the width w.

In exemplary embodiments, thickness t of cover glass sheet 500 is lessthan length l of cover glass sheet 500. In other exemplary embodiments,thickness t of cover glass sheet 500 is less than width w of cover glasssheet 500. In yet other exemplary embodiments, thickness t of coverglass sheet 500 is less than both length l and width w of cover glasssheet 500. As shown in FIG. 5 , cover glass sheet 500 further hasregions of permanent thermally induced compressive stress 530 and 540 atand/or near the first and second major surfaces 510, 520, balanced by aregion of permanent thermally induced central tensile stress 550 (i.e.,tension) in the central portion of the sheet.

The methods and systems may be used to form strengthened cover glasssheets having a wide variety of thickness ranges. In variousembodiments, thickness t of cover glass sheet 500 ranges from 0.1 mm to5.7 or 6.0 mm, including, in addition to the end point values, 0.2 mm,0.28 mm, 0.4 mm, 0.5 mm, 0.55 mm, 0.7 mm, 1 mm, 1.1 mm, 1.5 mm, 1.8 mm,2 mm, and 3.2 mm. Contemplated embodiments include thermallystrengthened cover glass sheets 500 having thicknesses tin ranges from0.1 to 20 mm, from 0.1 to 16 mm, from 0.1 to 12 mm, from 0.1 to 8 mm,from 0.1 to 6 mm, from 0.1 to 4 mm, from 0.1 to 3 mm, from 0.1 to 2 mm,from 0.1 to less than 2 mm, from 0.1 to 1.5 mm, from 0.1 to 1 mm, from0.1 to 0.7 mm, from 0.1 to 0.5 mm and from 0.1 to 0.3 mm.

In some embodiments, cover glass sheets of 3 mm or less in thickness areused. In some embodiments, the cover glass thickness is about (e.g.,plus or minus 1%) 8 mm or less, about 6 mm or less, about 3 mm or less,about 2.5 mm or less, about 2 mm or less, about 1.8 mm or less, about1.6 mm or less, about 1.4 mm or less, about 1.2 mm or less, about 1 mmor less, about 0.8 mm or less, about 0.7 mm or less, about 0.6 mm orless, about 0.5 mm or less, about 0.4 mm or less, about 0.3 mm or less,or about 0.28 mm or less.

In some embodiments, thermally strengthened cover glass sheets have highaspect ratios—i.e., the length and width to thickness ratios are large.Because the thermal tempering processes discussed herein do not rely onhigh pressures or large volumes of air, various cover glass sheetproperties, such as surface roughness and flatness, can be maintainedafter tempering by the use of gas bearings and high thermal transferrate systems discussed herein. Similarly, the thermal temperingprocesses discussed herein allow high aspect ratio cover glass sheets(i.e., cover glass sheets with high ratio of length to thickness, or ofwidth to thickness, or both) to be thermally strengthened whileretaining the desired or necessary shape. Specifically, sheets withlength to thickness and/or width to thickness ratios (“aspect ratios”)of approximately at least 10:1, at least 20:1, and up to and over 1000:1can be strengthened. In contemplated embodiments, sheets with aspectratios of at least 200:1, at least 500:1, at least 1000:1, at least2000:1, at least 4000:1 can be strengthened.

According to an exemplary embodiment, the length l of the strengthenedcover glass or glass-ceramic sheet 500 is greater than or equal to thewidth w, such as greater than twice the width w, greater than five timesthe width w, and/or no more than fifty times the width w. In some suchembodiments, the width w of the strengthened cover glass orglass-ceramic sheet 500 is greater than or equal to the thickness t,such as greater than twice the thickness t, greater than five times thethickness t, and/or no more than fifty times the thickness t.

In some embodiments, such as for applications disclosed with regard toFIGS. 27-30 discussed below, for example, the length l of the coverglass or glass-ceramic sheet 500 is at least 1 cm, such as at least 3cm, at least 5 cm, at least 7.5 cm, at least 20 cm, at least 50 cm,and/or no more than 50 m, such as no more than 10 m, no more than 7.5 m,no more than 5 m. In some such embodiments, the width w of the coverglass or glass-ceramic sheet 500 is at least 1 cm, such as at least 3cm, at least 5 cm, at least 7.5 cm, at least 20 cm, at least 50 cm,and/or no more than 50 m, such as no more than 10 m, no more than 7.5 m,no more than 5 m. Referring to FIG. 4 , cover glass or glass-ceramic isin the form a sheet 500 has a thickness t that is thinner than 5 cm,such as 2.5 cm or less, 1 cm or less, 5 mm or less, 2.5 mm or less, 2 mmor less, 1.7 mm or less, 1.5 mm or less, 1.2 mm or less, or even 1 mm orless in contemplated embodiments, such as 0.8 mm or less; and/or thethickness t is at least 10 μm, such as at least 50 μm, at least 100 μm,at least 300 μm.

In other contemplated embodiments, the cover glass or glass-ceramicarticle may be sized other than as disclosed herein. In contemplatedembodiments, the length l, width w, and/or thickness t of the coverglass or glass-ceramic articles may vary, such as for more complexgeometries (see generally FIG. 30 ), where dimensions disclosed hereinat least apply to aspects of the corresponding cover glass orglass-ceramic articles having the above-described definitions of lengthl, width w, and thickness t with respect to one another.

In some embodiments, at least one of the first or second surfaces 510,520 of cover glass sheet 500 has a relatively large surface area. Invarious embodiments, first and/or second surfaces 510, 520 having areasof at least 100 mm², such as at least 900 mm², at least 2500 mm², atleast 5000 mm², at least 100 cm², at least 900 cm², at least 2500 cm²,at least 5000 cm², and/or no more than 2500 m², such as no more than 100m², no more than 5000 cm², no more than 2500 cm², no more than 1000 cm²,no more than 500 cm², no more than 100 cm². As such, the cover glass orglass-ceramic sheet 500 may have a relatively large surface area; which,except by methods and systems disclosed herein, may be difficult orimpossible to thermally strengthen particularly while having thethicknesses, surface qualities, and/or strain homogeneities of the coverglass sheets discussed herein. Further, except by methods and systemsdisclosed herein, it may be difficult or impossible to achieve thestress profile, particularly the negative tensile stress portion of thestress profile (see generally FIG. 6 ), without relying uponion-exchange or a change in the type of cover glass.

Thermally Strengthened Cover Glass Sheet Compressive and TensileStresses

As noted above, the thermally strengthened cover glass sheets discussedherein may have surprisingly high surface compressive stresses, e.g., inregions 530, 540 shown in FIG. 5 , surprisingly high central tensilestresses, e.g., in region 550 shown in FIG. 5 , and/or unique stressprofiles (see FIG. 6 ). This is particularly true considering the lowthickness and/or other unique physical properties (e.g., very lowroughness, high degree of flatness, various optical properties, fictivetemperature properties, etc.) of cover glass sheet 500 as discussedherein.

Compressive stresses of cover glasses (e.g., in regions 530, 540 shownin FIG. 5 ) formed by the processes and systems disclosed herein canvary as a function of thickness t of the cover glasses. In variousembodiments, cover glasses, e.g., cover glass sheet 500, having athickness of 3 mm or less have a compressive stress (e.g., surfacecompressive stress) of at least 80 MPa, at least 100 MPa, at least 150MPa, at least 200 MPa, at least 250 MPa, at least 300 MPa, at least 350MPa, at least 400 MPa, and/or no more than 1 GPa. In contemplatedembodiments, cover glasses having a thickness of 2 mm or less have acompressive stress of at least 80 MPa, at least 100 MPa, at least 150MPa, at least 175 MPa, at least 200 MPa, at least 250 MPa, at least 300MPa, at least 350 MPa, at least 400 MPa, and/or no more than 1 GPa. Incontemplated embodiments, cover glasses having a thickness of 1.5 mm orless have a compressive stress of at least 80 MPa, at least 100-MPa, atleast 150 MPa, at least 175 MPa, at least 200 MPa, at least 250 MPa, atleast 300-MPa, at least 350 MPa, and/or no more than 1 GPa. Incontemplated embodiments, cover glasses having a thickness of 1 mm orless have a compressive stress of at least 80 MPa, at least 100 MPa, atleast 150 MPa, at least 175 MPa, at least 200 MPa, at least 250 MPa, atleast 300 MPa, and/or no more than 1 GPa. In contemplated embodiments,cover glasses having a thickness of 0.5 mm or less have a compressivestress of at least 50 MPa, at least 80 MPa, at least 100 MPa, at least150 MPa, at least 175 MPa, at least 200 MPa, at least 250 MPa, and/or nomore than 1 GPa.

In some embodiments, the thermally induced central tension in coverglasses formed by the processes and systems disclosed herein (e.g., inthe region 550 shown in FIG. 5 ) may be greater than 40 MPa, greaterthan 50 MPa, greater than 75 MPa, greater than 100 MPa. In otherembodiments, the thermally induced central tension may be less than 300MPa, or less than 400 MPa. In some embodiments, the thermally inducedcentral tension may be from about 50 MPa to about 300 MPa, about 60 MPato about 200 MPa, about 70 MPa to about 150 MPa, or about 80 MPa toabout 140 MPa. In some embodiments, the thermally strengthened coverglass sheets have high thinness i.e., are particularly thin. Becausevery high-heat transfer rates can be applied via the systems and methodsdiscussed herein, significant thermal effects, for example centraltensions of at least 10 or even at least 20 MPa, can be produced insheets of SLG of less than 0.3 mm thickness. In fact, very thin sheets,sheets at least as thin as 0.1 mm, can be thermal strengthened. Specificlevels of thermal stresses achieved and achievable, considered as afunction of thickness and other variables, are described in furtherdetail herein.

Referring to FIG. 6 , a conceptual stress profile 560, at roomtemperature of 25° C. and standard atmospheric pressure, of thestrengthened cover glass or glass-ceramic sheet 500 of FIG. 4 , shows aninterior portion 550 of the strengthened cover glass or glass-ceramicsheet 500 under positive tensile stress and portions 530, 540 of thestrengthened cover glass or glass-ceramic sheet 500 exterior to andadjoining the interior portion 550 under negative tensile stress (e.g.,positive compressive stress). Applicant believes that the negativetensile stress at least in part fortifies the strengthened cover glassor glass-ceramic sheet 500 by limiting initiation and/or propagation ofcracks therethrough.

Believed unique to the present inventive technology, given relativelylarge surface areas and/or thin thicknesses of the strengthened coverglass or glass-ceramic sheet 500 as disclosed herein, tensile stress inthe stress profile 560 sharply transitions between the positive tensilestress of the interior portion 550 and the negative tensile stress ofthe portions 530, 540 exterior to and adjoining the interior portion550. This sharp transition may be understood as a rate of change (i.e.,slope) of the tensile stress which may be expressed as a magnitude ofstress (e.g., 100 MPa, 200 MPa, 250 MPa, 300 MPa, 400 MPa, a differencein peak values of the positive and negative tensile stresses +σ, −σ)divided by a distance of thickness over which the change occurs, such asa distance of 1 mm, such as a distance of 500 μm, 250 μm, 100 μm (whichis the distance used to quantify a rate of change, which may be aportion of article thickness, and not necessarily a dimension of thearticle geometry). In some such embodiments, the rate of change of thetensile stress does not exceed 7000 MPa divided by 1 mm, such as no morethan 5000 MPa divided by 1 mm. In contemplated embodiments, thedifference in peak values of the positive and negative tensile stressesis at least 50 MPa, such as at least 100 MPa, at least 150 MPa, at least200 MPa, at least 250 MPa, at least 300 MPa, at least 400 MPa, at least500 MPa, and/or no more than 50 GPa. In contemplated embodiments, thecover glass or glass-ceramic sheet 500 has a peak negative tensilestress of at least 50 MPa in magnitude, such as at least 100 MPa, atleast 150 MPa, at least 200 MPa, at least 250 MPa, at least 300 MPa, atleast 400 MPa, at least 500 MPa. The steep tensile curve transitionsgenerated by the system and method discussed herein are believed to beindicative of the ability to achieve higher magnitudes of negativetensile stress at a surface of a cover glass sheet for a given thicknessand/or to manufacture thinner cover glass articles to a higher degree ofnegative tensile stress, such as to achieve a fragmentation potentialfor dicing as disclosed herein. Conventional thermal temperingapproaches may be unable to achieve such steep tensile stress curves.

According to an exemplary embodiment, the high rate of change of tensilestress is at least one of the above-described magnitudes or greatersustained over a thickness-wise stretch of the stress profile 560 thatis at least 2% of the thickness, such as at least 5% of the thickness,at least 10% of the thickness, at least 15% of the thickness, or atleast 25% of the thickness of cover glass sheet 500. In contemplatedembodiments, the strengthening extends deep into the strengthened coverglass or glass-ceramic sheet 500 such that the thickness-wise stretchwith the high rate of change of tensile stress is centered at a depth ofbetween 20% and 80% into the thickness from the first surface, which mayfurther distinguish chemical tempering for example.

In at least some contemplated embodiments, the strengthened cover glassor glass-ceramic article includes a change in the composition thereof interms of ion content, conceptually shown as dotted line 562 in FIG. 6 .More specifically, the composition of the strengthened cover glass orglass-ceramic article 500 in such embodiments includes exchanged orimplanted ions that influence the stress profile 560. In some suchembodiments, the exchanged or implanted ions do not extend fully throughthe portions 530, 540 of the strengthened cover glass or glass-ceramicarticle 500 under the negative tensile stress because the negativetensile stress is also a result of the thermal tempering as disclosedherein.

Accordingly, the curve of the tensile stress profile 560 with ionexchange strength augmentation includes a discontinuity or sudden change564 in direction where tangents of the curve differ from one another oneither side of the discontinuity or sudden change 564. The sudden change564 is located within the portions 530, 540 under negative tensilestress such that the tensile stress is negative on either sideimmediately adjacent to the discontinuity or sudden change 564. Thediscontinuity or sudden change 564 may correspond to the depth of thedifferent ion content, however in some such embodiments other parts ofthe portions 530, 540 under negative tensile stress still have the samecomposition in terms of ion content as the portion 550 under positivetensile stress.

Put another way, for at least some strengthened cover glass or glassceramic articles 500, with or without ion-exchange or implantation, thecomposition of at least a part of the portions 530, 540 of thestrengthened cover glass or glass-ceramic sheet 500, which is under thenegative tensile stress and is exterior to and adjoining the interiorportion 550, is the same as the composition of at least a part of theinterior portion 550, which is under the positive tensile stress. Insuch embodiments, at least some of the negative tensile stress of thestress profile is independent of a change in the composition (e.g., ioncomposition) of the strengthened cover glass or glass-ceramic sheet 500.Such structure may simplify the composition of the strengthened coverglass or glass-ceramic sheet 500 at least to a degree by providingsufficient strength without and/or with less chemical tempering.Further, such structure may reduce stress concentrations within thestrengthened cover glass or glass-ceramic sheet 500 due todiscontinuity/changes in composition, possibly reducing chances ofdelamination and/or cracking at the composition discontinuity.

Thermally Tempered Cover Glass Sheet Break Performance

If sufficient energy is stored in the region of tensile stress 550, thecover glass will break like safety glass or “dice” when sufficientlydamaged. As used herein, a cover glass sheet is considered to dice whenan area of the cover glass sheet 25 cm² breaks into 40 or more pieces.In some embodiments, dicing is used as a qualitative measure of showingthat the cover glass sheet is “fully tempered” (i.e., for 2 mm orthicker cover glass, where the cover glass sheet has a compressivestress of at least 65 MPa or an edge compression of at least 67 MPa). Invarious embodiments, cover glass sheet 500 has sufficient tensile stressin region of tensile stress 550 such that a 25 cm² piece of cover glasssheet 500 breaks into 40 or more pieces.

Referring to FIG. 7 , a cover glass or glass-ceramic article 610, havingproperties as disclosed herein with respect to the cover glass orglass-ceramic sheets, such as sheet 500, has been fractured, such asusing a prick punch or other instrument and/or generally in accordancewith American National Standards Institute (ANSI) Z97.1 (impact test)and the ASTM 1048 standard. According to an exemplary embodiment, thecover glass or glass ceramic article 610 has been strengthened to adegree that dicing has occurred upon the fracture, forming a pluralityof small granular chunks 616 (e.g., fragments, pieces). In someembodiments, the cover glass or glass-ceramic article 610 has athermally-induced stress sufficient to produce a number of granularchunks 616 that is not less than 40 within an area of 50-by-50 mm of thecover glass or glass-ceramic article 610 in a fragmentation test inwhich an impact is applied with a hammer or a punch to initiate crackingof the cover glass into granular pieces. A standard office thumb tack612, with a metal pin length 614 of about 1 cm is shown for reference.

According to various contemplated embodiments, despite the thinthickness of the strengthened cover glass or glass-ceramic article 610,the stress profile (see generally FIG. 6 ) imparts a high fragmentationpotential of the strengthened cover glass or glass-ceramic article 610such that when fractured the strengthened cover glass or glass-ceramicarticle 610 shatters into particularly small granular chunks 616, thosehaving an area on either the first or second surface of less than 90mm², such as less than 50 mm², such as less than 20 mm², such as lessthan 10 mm², such as less than 5 mm², and/or at least 10 μm². In somesuch embodiments, the fragmentation potential of the strengthened coverglass or glass-ceramic article 610 is such that at least 20% (e.g., atleast 50%, at least 70%, at least 95%) of the granular chunks 616 havean area of at least one of the first or second surfaces of one of theabove-described amounts when the strengthened cover glass orglass-ceramic article is fractured.

Due at least in part to the particularly thin geometry of the coverglass or glass-ceramic article 610 that may be manufactured with thetensile stresses as disclosed herein using the inventive technology insome embodiments, the fragmentation potential of the strengthened coverglass or glass-ceramic article 610 is such that, when fractured, thestrengthened cover glass or glass-ceramic article 610 shatters intoparticularly low-volume granular chunks, those having a volume of lessthan 50 mm³, such as less than 40 mm³, such as less than 30 mm³, such asless than 25 mm³, and/or at least a volume of 50 μm³.

Due at least in part to the particularly large area of the cover glassor glass-ceramic article 610 that may be manufactured with the tensilestresses as disclosed herein using the inventive technology in someembodiments, the fragmentation potential of the strengthened cover glassor glass-ceramic article 610 is such that, when fractured, thestrengthened cover glass or glass-ceramic article 610 shatters into atleast 100 granular chunks 616 of at least of 50 μm³ in volume, such asat least 200, at least 400, at least 1000, at least 4000 granular chunks616 of at least of 50 μm³ in volume.

Referring now to FIG. 8 and FIG. 9 , experiments were performed on 1.1mm thick glass sheets of glass comprising at least 70% silicon dioxideby weight, and/or at least 10% sodium oxide by weight, and/or at least7% calcium oxide by weight, and strengthened using the equipment andprocesses disclosed herein. As shown in FIG. 8 , the number of granularchunks 616 per square centimeter of the glass has been found to begenerally related to the magnitude of positive tensile stress at thecenter of the respective cover glass or glass-ceramic article 610.Similarly, as shown in FIG. 9 , the fragmentation potential of therespective cover glass or glass-ceramic article 610 has also been foundto be related to temperature of the glass in the hot zone (see e.g.,FIG. 21 , FIG. 22 and FIG. 23 ) and the calculated expected heattransfer coefficient (h) in units of cal/cm²·s·° C. (SI units watt/m²·°K) effectively applied to the cover glass surfaces during quenching,based on size of the gap between the glass sheet surfaces and the heatsink/gas bearing during quenching and on the thermal conductivity of thegas used in the gap.

Thermally Tempered Cover Glass Sheet Fictive Temperature

In various embodiments, the thermally strengthened cover glass sheetsformed by the systems and methods discussed herein (e.g., cover glasssheet 500) have high fictive temperatures. It will be understood that invarious embodiments, high fictive temperatures of the cover glassmaterials discussed herein relate to the high level of tempering, highcentral tensile stresses and/or high compressive surface stress of coverglass sheet 500. Surface fictive temperatures may be determined by anysuitable method, including differential scanning calorimetry, Brillouinspectroscopy, or Raman spectroscopy.

According to an exemplary embodiment, the cover glass or glass-ceramicsheet 500 has a portion thereof, such as at or near the first and/orsecond surfaces 510, 520, that has a particularly high fictivetemperature, such as at least 500° C., such as at least 600° C., or evenat least 700° C. in some embodiments, such as for soda-lime glass.According to an exemplary embodiment, the cover glass or glass-ceramicsheet 500 has a portion thereof, such as at or near the first and/orsecond surfaces 510, 520, that has a particularly high fictivetemperature relative to annealed glass of the same chemical composition,such as at least 10° C. greater, at least 30° C. greater, at least 50°C. greater, at least 70° C. greater, or even at least 100° C. greater.High fictive temperature may be achieved by the presently disclosedinventive technology at least in part due to the rapid transition fromthe hot to the cooling zones in the strengthening system (see e.g., FIG.21 , FIG. 22 and FIG. 23 ). Applicant believes that high fictivetemperature may correspond or relate to increased damage resistance ofglass.

In some methods of determining surface fictive temperatures, it may benecessary to break the glass to relieve the “temper stresses” induced bythe heat strengthening process in order to measure fictive temperaturewith reasonably accuracy. It is well known that characteristic structurebands measured by Raman spectroscopy shift in a controlled manner bothwith respect to the fictive temperature and with respect to appliedstress in silicate glasses. This shift can be used to non-destructivelymeasure the fictive temperature of a thermally strengthened cover glasssheet if the temper stress is known.

Referring generally to FIG. 10 , determination of fictive temperaturefor several exemplary cover glass articles is shown. Stress effects onthe Raman spectrum of silica glass are reported in D. R. Tallant, T. A.Michalske, and W. L. Smith, “The effects of tensile stress on the Ramanspectrum of silica glass,” J. Non-Cryst. Solids, 106 380-383 (1988).Commercial glasses of 65 wt. % silica or more have substantially thesame response. Although the reported stress response is for uniaxialstress, in the case of a unibiaxial stress state such as that which isobserved in tempered glass, σ_(xx)=σ_(yy), the peak can be expected toshift by twice that expected by a uniaxial stress. The peak near 1090cm⁻¹ in soda-lime glass and in glass 2 corresponds to the 1050 cm⁻¹ peakobserved in silica glass. The effects of stress on the 1050 cm⁻¹ peak insilica, and on the corresponding peak in SLG and other silicate glassescan be expressed, as a function of stress σ in MPa, by equation a)ω(cm⁻¹)=1054.93−0.00232·σ.

A calibration curve was produced of Raman band positions as a functionof the fictive temperature for SLG and another glass, glass 2. Glasssamples were heat-treated for various times, 2-3 times longer than thestructural relaxation times calculated by τ=10*η/G, where η is theviscosity, and G the shear modulus. After heat-treatment, the glasseswere quenched in water to freeze the fictive temperature at theheat-treatment temperature. The glass surfaces were then measured bymicro Raman at 50× magnification and a 1-2 μm spot size using a 442 nmlaser, 10-30 s exposure time, and 100% power, over the range of 200-1800cm⁻¹. The position of the peak at 1000-1200 cm⁻¹ was fit using computersoftware, Renishaw WiRE version 4.1, in this case. A good fit of the1090 cm⁻¹ Raman peak measured in SLG on the air side as a function offictive temperature Tf (in ° C.) is given by equation b)ω(cm⁻¹)=1110.66−0.0282·Tf. For glass 2, a good fit is given by equationc) ω(cm⁻¹)=1102.00−0.0231·Tf.

Using the relationships established in equations a), b), and c), it ispossible to express the fictive temperature of the cover glass as afunction of a measured Raman peak position with a correction factor dueto surface compressive stress. A compressive stress of 100 MPa, σ_(c),shifts the Raman band position equivalent to approximately a 15 to 20degree Celsius reduction in the fictive temperature. The followingequation is applicable to SLG:

$\begin{matrix}{{T_{f}\left( {{^\circ}{C.}} \right)} = {\left\lbrack \frac{{\omega\left( {cm}^{- 1} \right)} - {111{0.6}6\left( {cm}^{- 1} \right)}}{{- {0.0}}282\left( \frac{{cm}^{- 1}}{{^\circ}{C.}} \right)} \right\rbrack + {2\left\lbrack {{0.0}82*{\sigma_{c}({MPa})}} \right\rbrack}}} & (1)\end{matrix}$

The equation applicable to glass 2 is:

$\begin{matrix}{{T_{f}\left( {{^\circ}{C.}} \right)} = {\left\lbrack \frac{{\omega\left( {cm}^{- 1} \right)} - {1102\left( {cm}^{- 1} \right)}}{{- {0.0}}231\left( \frac{{cm}^{- 1}}{{^\circ}{C.}} \right)} \right\rbrack + {2\left\lbrack {{0.0}996*{\sigma_{c}({MPa})}} \right\rbrack}}} & (2)\end{matrix}$

In these equations, ω is the measured peak wavenumber for the peak near1090 cm⁻¹, σ_(c) is the surface compressive stress measured by anysuitable technique, yielding stress-corrected measurement of fictivetemperature in ° C. As a demonstration of increased damage resistancerelated to the determined fictive temperature, four glass sheet sampleswere prepared, two 6 mm soda-lime glass (SLG) sheets by conventionaltempering methods to approximately 70 and 110 MPa surface compressivestress (CS), and two 1.1 mm SLG sheets by the methods and systemsdisclosed herein to about the same levels of CS. Two additional sheets,one of each thickness were used as controls. The surfaces of each testsheet were subjected to standard Vickers indentation. Various levels offorce were applied, for 15 seconds each, and after a 24 hour wait,indentations were each examined. As shown in Table I, the 50% crackingthreshold (defined as the load at which the average number of cracksappearing is two out of the four points of the indenter at which crackstend to initiate) was determined for each sample.

Table I shows that the Vickers crack initiation threshold for SLGprocessed by conventional convective gas tempering (as reflected in the6 mm sheet) is essentially the same as that for annealed or as-deliveredSLG sheets, rising from between zero and one newton (N) to about one toless than two newtons. This correlates with the relatively modest risein surface fictive temperature (T_(fs) or Tf_(surface)) of ˜25 to 35° C.relative to glass transition temperature (T_(g)=550° C. for SLG, definedas η=10^(12-13.3) Poise) that was provided by conventional tempering. Incontrast, by tempering using the present methods and systems, theVickers crack initiation threshold improved to greater than 10 N, a10-fold increase over the Vickers damage resistance imparted byconventional tempering. In the embodied glasses, the T_(fs) minus T_(g)was at least 50° C., or at least 75° C., or at least 90° C., or in therange of from approximately 75° C. to 100° C. Even in embodimentscomprising lower levels of heat strengthening, the embodied glasses canstill provide increased resistance, at levels such as 5 N, for instance.In certain contemplated embodiments, the 50% cracking threshold after a15 second Vickers crack initiation test may be equal to or greater than5 N, 10 N, 20 N, or 30 N.

TABLE I Thickness CS Surface T_(f) Cracking Sample (mm) (MPa) (° C.)Threshold (N) Control 1.1 Annealed ~T_(g) (550) 0-1 Control 6 Annealed~T_(g) (550) 0-1 Thin low strength 1.1 −72 626 10-20 Thick low strength6 −66 575 1-2 Thin medium strength 1.1 −106 642 10-20 Thick mediumstrength 6 −114 586 1-2

The following non-dimensional fictive temperature parameter θ can beused to compare the relative performance of a thermal strengtheningprocess in terms of the fictive temperature produced. Given in terms ofsurface fictive temperature θs in this case:θs=(T _(fs) −T _(anneal))/(T _(soft) −T _(anneal))  (3)where T_(fs) is the surface fictive temperature, T_(anneal) (thetemperature of the glass at a viscosity of η=10^(13.2) Poise) is theannealing point and T_(soft) (the temperature of the glass at aviscosity of η=10^(7.6) Poise) is the softening point of the glass ofthe sheet. FIG. 10 is a plot of θs for measured surface fictivetemperatures as a function of heat transfer rate, h, applied duringthermal strengthening for two different glasses. As shown in FIG. 10 ,the results for the two different glasses overlie each other fairlyclosely. This means that parameter θ provides a means to compare thefictive temperatures of different glasses compared directly, in relationto the heat transfer rate h required to produce them. The vertical rangeof results at each h corresponds to variation in the value of T₀, theinitial temperature at the start of quenching. In embodiments, parameterθs comprises from about (e.g., plus or minus 10%) 0.2 to about 0.9, or0.21 to 0.09, or 0.22 to 0.09, or 0.23 to 0.09, or 0.24 to 0.09, or 0.25to 0.09, or 0.30 to 0.09, or 0.40 to 0.09, or 0.5 to 0.9, or 0.51 to0.9, or 0.52 to 0.9, or 0.53 to 0.9, or 0.54 to 0.9, or 0.54 to 0.9, or0.55 to 0.9, or 0.6 to 0.9, or even 0.65 to 0.9.

Thermally Tempered Cover Glass Sheet Temperability Parameter

In various embodiments, the thermally strengthened cover glass sheetsformed by the systems and methods discussed herein (e.g., cover glasssheet 500) have a high temperability and/or heat transfer value. The“specific thermal stress” of a glass is given by:

$\begin{matrix}\frac{\alpha \cdot E}{1 - \mu} & (4)\end{matrix}$where α is the (low temperature linear) CTE of the glass, E is themodulus of elasticity of the glass material and μ is Poisson's ratio ofthe glass material. This value is used to indicate the level of stressproduced within a given glass composition when subjected to atemperature gradient. It may also be used as an estimator of thermal“temperability.” At higher thermal transfer rates (such as at about 800W/m²K and above, for example), however, the high temperature or“liquidus” CTE of the glass begins to affect tempering performance.Therefore, under such conditions, the temperability parameter Ψ, basedon an approximation of integration over the changing CTE values acrossthe viscosity curve, is found to be useful:Ψ=E·[T _(strain) ·αC _(CTE) ^(S)+α_(CTE) ^(L)·(T _(soft) −T_(strain))]  (5)where α^(S) _(CTE) is the low temperature linear CTE (equivalent to theaverage linear expansion coefficient from 0-300° C. for the glass),expressed in 1/° C. (° C.⁻¹), α^(L) _(CTE) is the high temperaturelinear CTE (equivalent to the high-temperature plateau value which isobserved to occur somewhere between the glass transition and softeningpoint), expressed in 1/° C. (° C.⁻¹), E is the elastic modulus of theglass, expressed in GPa (not MPa) (which allows values of the(non-dimensional) parameter to range generally between 0 and 1),T_(strain) is the strain point temperature of the glass, (thetemperature of the glass at a viscosity of η=10^(14.7) Poise) expressedin ° C., and T_(soft) is the softening point of the glass (thetemperature of the glass at a viscosity of η=10^(7.6) Poise), expressedin ° C.

The thermal strengthening process and resulting surface compressivestresses were modeled for glasses having varying properties to determinethe tempering parameter, Ψ. The glasses were modeled at the samestarting viscosity of 10^(8.2) Poise and at varying heat transfercoefficients. The properties of the various glasses are shown in TableII, together with the temperature for each glass at 10^(8.2) Poise andthe calculated value of the temperability parameter Ψ for each.

TABLE II 10^(8.2) Softening Strain CTE CTE Poise Point Point GlassModulus low high ° C. ° C. ° C. Ψ SLG 72 8.8 27.61 705 728 507 0.76 273.3 8.53 20.49 813 837 553 0.77 3 65.5 8.26 26 821 862 549 0.83 4 658.69 20.2 864 912 608 0.74 5 63.9 10.61 22 849 884 557 0.84 6 58.26 3.520.2 842 876 557 0.49 7 73.6 3.6 13.3 929 963 708 0.44 8 81.1 3.86 12.13968 995 749 0.48

The results in Table II show that is proportional to the thermalstrengthening performance of the glass. This correlation is furthershown in FIG. 11 , which provides an embodied example for a high heattransfer rate (a heat transfer coefficient of 2093 W/m²K (0.05cal/s·cm²·° C.)) and a glass sheet thickness of only 1 mm. As seen inthe figure, the variation in the seven differing glasses' resultingcompressive stress correlates well with the variation in the proposedtemperability parameter Ψ.

Thermally Tempered Cover Glass Sheet Heat Transfer Coefficient andRelation to Surface Compressive Stress and Central Tension Stress

In another aspect, it has been found that for any glass, at any givenvalue of the heat transfer coefficient, h (expressed in cal/cm²−s−° C.),the curves of surface compressive stress (σ_(CS), in MPa) vs. thickness(t, in mm) can be fit (over the range oft from 0 to 6 mm) by thehyperbola, where P₁ and P₂ are functions of h such that:

$\begin{matrix}{{\sigma_{CS}\left( {{Glass},h,t} \right)} = {{{C\left( {h,t} \right)}*{\Psi({Glass})}} = {\frac{{P_{1}(h)}*t}{\left( {{P_{2}(h)} + t} \right)}*{\Psi({Glass})}}}} & (6)\end{matrix}$or with the expression for Ψ substituted in, the curve of compressivestress σ_(CS) (Glass,h,t) is given by:

$\begin{matrix}{\frac{{P_{1}(h)}*t}{\left( {{P_{2}(h)} + t} \right)} \cdot E \cdot \left\lbrack {{T_{strain} \cdot \alpha_{CTE}^{s}} + {\alpha_{CTE}^{L} \cdot \left( {T_{soft} - T_{strain}} \right)}} \right\rbrack} & (7)\end{matrix}$where the constants P₁, P₂, in either (6) or (7) above, are eachcontinuous functions of the heat transfer value, h, given by:

$\begin{matrix}{P_{1} = {{91{0.2}} - {259.2 \cdot {\exp\left( {- \frac{h}{{0.1}43}} \right)}}}} & (8)\end{matrix}$ $\begin{matrix}{and} & \end{matrix}$ $\begin{matrix}{P_{2} = {{{2.5}3} + \frac{2{3.6}5}{\left( {1 + \left( \frac{h}{{0.0}0738} \right)^{{1.5}8}} \right)}}} & (9)\end{matrix}$The constants P₁, P₂, are graphed as functions of h in FIGS. 12 and 13 ,respectively. Accordingly, by using a value of P₁, for a given h and thecorresponding P₂, for that same h in expression (6) or (7) above, acurve is specified corresponding to the surface compressive stress (CS)obtainable at that h, as a function of thickness t.

In some embodiments, a similar expression may be used to predict thecentral tension (CT) of a thermally strengthened cover glass sheet,particularly at a thickness of 6 mm and less, and the thermal transfercoefficient, such as 800 W/m²K and up, by simply dividing thecompressive stress predicted under the same conductions by 2. Thus,expected central tension may be given by

$\begin{matrix}{\frac{{P_{1CT}\left( h_{CT} \right)}*t}{\left( {{P_{2CT}\left( h_{CT} \right)} + t} \right)} \cdot E \cdot \left\lbrack {{T_{strain} \cdot \alpha_{CTE}^{s}} + {\alpha_{CTE}^{L} \cdot \left( {T_{soft} - T_{strain}} \right)}} \right\rbrack} & (10)\end{matrix}$Where P_(1CT) and P_(2CT) are given as follows:

$\begin{matrix}{P_{1CT} = {{91{0.2}} - {259.2 \cdot {\exp\left( {- \frac{h_{CT}}{{0.1}43}} \right)}}}} & (11)\end{matrix}$ $\begin{matrix}{and} & \end{matrix}$ $\begin{matrix}{P_{2CT} = {{{2.5}3} + \frac{2{3.6}5}{\left( {1 + \left( \frac{h_{CT}}{{0.0}0738} \right)^{{1.5}8}} \right)}}} & (12)\end{matrix}$In some embodiments, h and h_(CT), may have the same value for a givenphysical instance of thermal strengthening. However, in someembodiments, they may vary, and providing separate variables andallowing variation between them allows for capturing, within descriptiveperformance curves, instances in which the typical ratio of 2:1 CS/CTdoes not hold.

One or more embodiments of the currently disclosed processes and systemshave produced thermally strengthened SLG sheets at all of the heattransfer rate values (h and h_(CT)) shown in Table III.

TABLE III h and h_(CT) values according to exemplary embodiments cal/s ·cm² · ° C. W/m²K 0.010 418.68 0.013 544.284 0.018 753.624 0.019 795.4920.020 837.36 0.021 879.228 0.022 921.096 0.023 962.964 0.027 1130.4360.028 1172.304 0.029 1214.172 0.030 1256.04 0.031 1297.908 0.0331381.644 0.034 1423.512 0.038 1590.984 0.040 1674.72 0.041 1716.5880.042 1758.456 0.045 1884.06 0.047 1967.796 0.048 2009.664 0.0492051.532 0.050 2093.4 0.051 2135.268 0.052 2177.136 0.053 2219.004 0.0542260.872 0.055 2302.74 0.060 2512.08 0.061 2553.948 0.062 2595.816 0.0632637.684 0.065 2721.42 0.067 2805.156 0.069 2888.892 0.070 2930.76 0.0712972.628 0.078 3265.704 0.080 3349.44 0.081 3391.308 0.082 3433.1760.095 3977.46 0.096 4019.328 0.102 4270.536 0.104 4354.272 0.105 4396.140.127 5317.236 0.144 6028.992 0.148 6196.464 0.149 6238.332 0.1847703.712

In some embodiments, the heat transfer value rates (h and k_(CT)) may befrom about 0.024 to about 0.15, about 0.026 to about 0.10, or about0.026 to about 0.075 cal/s·cm²·° C.

FIG. 14 shows the newly opened performance space in MPa of surfacecompression of a glass sheet as a function of thickness t (in mm), by agraph of C(h,t)·Ψ(SLG) for selected values of h according to equations6-9 above, with Ψ(SLG) corresponding to the value of Ψ for SLG in TableII. The traces labeled GC represent the estimated range of maximumstresses versus thinness of SLG sheets achievable by gas convectivetempering, from 0.02 cal/scm²° C. (or 840 W/m²K) to 0.03 cal/s·cm²·° C.or 1250 W/m²K, assuming that these levels of heat transfer coefficientcan be employed in that process at a heated glass viscosity of 10^(8.2)Poises or about 704° C., a temperature above the capability ofconvective gas processes.

Examples of highest reported sheet CS values based on gas convectivetempering processes are shown by the triangle markers labeled Gas in thelegend. The value 601 represents advertised product performancecapability of commercial equipment, while the value 602 is based on anoral report at a glass processing conference. The trace labeled LCrepresents the curve of maximum stresses versus thinness of SLG sheetsestimated to be achievable by liquid contact tempering, given by a heattransfer rate h of 0.0625 cal/s·cm²·° C. (or about 2600 W/m²K), alsoassuming processing at an initial heated glass viscosity of 10^(8.2)Poises or about 704° C. Examples of highest reported sheet CS valuesbased on liquid contact tempering processes are shown by the circlemarkers labeled Liquid in the legend. The higher of the two values at 2mm thickness is based on a report of tempering of a borosilicate coverglass sheet, and the stress achieved has been scaled for the figure by(Ψ_(SLG))/(Ψ_(borosilicate)) for scaled direct comparison.

The trace labeled 704 represents stresses achievable by one or moreembodiments of the presently disclosed methods and systems at a heattransfer rate of 0.20 cal/s·cm²·° C. (or about 8370 W/m²K) and aninitial temperature, just before quenching, of 704° C. The level ofstress on the cover glass sheet thus achievable represents almost thesame scope of improvement over liquid tempering strength levels asliquid tempering represents over state of the art gas convectivetempering. But the trace labeled 704 is not an upper limit—embodimentshave been shown to be viable above this value due to the good control ofform and flatness achievable in a small-gap gas bearing thermalstrengthening at even higher temperatures (at lower viscosities of thecover glass). The trace labeled 730 shows some of the additionalstrengthening performance achieved by a heat transfer rate of 0.20cal/s·cm²° C. (or about 8370 W/m²K) at a starting temperature for a SLGsheet of 730° C., very near or above the softening point of the coverglass. Significant improvements in compressive stress and thus in coverglass sheet strength are thus achieved particularly by the combinationof high heat transfer rate and the use of high initial temperaturesenabled by the good handling and control of sheet flatness and form in atight gas bearing—and the improvements are particularly striking atthickness 2 mm and below.

FIG. 15 shows the traces of FIG. 14 explained above, at 2 mm and below,but with compressive stress as a function of thickness plotted forselected examples of tempered cover glass sheets produced by one or moreembodiments of the present disclosure, showing the extreme combinationof thermal strengthening levels and thinness enabled by the presentdisclosure.

Thermally Tempered Cover Glass Sheet with Low Surface Roughness and HighDegree of Flatness

In various embodiments, thermally strengthened cover glass sheetsdisclosed herein, such as sheet 500, have both high thermal stresses andlow, as-formed surface roughness. The processes and methods disclosedherein can thermally strengthen a sheet of cover glass withoutincreasing the surface roughness of the as-formed surfaces. For example,incoming float cover glass air-side surfaces and incoming fusion formedcover glass surfaces were characterized by atomic force microscopy (AFM)before and after processing. R_(a) surface roughness was less than 1 nm(0.6-0.7 nm) for incoming 1.1 mm soda-lime float cover glass, and theR_(a) surface roughness was not increased by thermal strengtheningaccording to the present processes. Similarly, an R_(a) surfaceroughness of less than 0.3 nm (0.2-0.3) for 1.1 mm sheets offusion-formed cover glass was maintained by thermal strengtheningaccording to this disclosure. Accordingly, thermally strengthened coverglass sheets have a surface roughness on at least a first surface in therange from 0.2 to 1.5 nm R_(a) roughness, 0.2 to 2.0 nm R_(a) roughness,0.2 to 0.7 nm, 0.2 to 0.4 nm or even such as 0.2 to 0.3 nm, over atleast an area of 10×10 μm. Surface roughness may be measured over anarea of 10×10 μm in exemplary embodiments, or in some embodiments, 15×15μm.

In some contemplated embodiments, thermally strengthened cover glasssheets disclosed herein have both high thermal stresses and low,as-formed surface roughness and/or coated surfaces. The processes andmethods disclosed herein can thermally strengthen a sheet of cover glasswithout increasing the surface roughness of smooth as-formed oras-delivered surfaces of cover glass sheets, and likewise withoutdamaging sensitive low-E or anti-reflective or other coatings. Incomingfloat cover glass air-side surfaces, and incoming fusion-formed coverglass surfaces, were characterized by atomic force microscopy (AFM)before and after processing. R_(a) surface roughness was less than 1 nm(such as 0.6 to 0.7 nm) for incoming on the air side of 1.1 mm soda-limefloat cover glass and was not increased by thermal strengtheningaccording to the present disclosure. R_(a) surface roughness was lessthan 0.3 nm (such as 0.2 to 0.3 nm) incoming on 1.1 mm sheets offusion-formed cover glass and likewise was not increased by thermalstrengthening according to this disclosure. Accordingly, in contemplatedembodiments, thermally strengthened cover glass sheets, according tothis disclosure, have surface roughness on at least a first surface inthe range of at least 0.2 nm and/or no more than 1.5 nm R_(a) roughness,such as no more than 0.7 nm, such as no more than 0.4 nm or even such asno more than 0.3 nm, or have thermally strengthened sheets havingcoatings thereon of the type that may be applied before strengthening,or have combinations of these low roughness values and coatings, areobtained from the present process used with corresponding cover glasssheets as starting material. It is Applicant's understanding that suchpreservation of surface quality and/or surface coating(s) previouslyrequired use of convective gas tempering or perhaps a low heat transferliquid tempering process, which produces limited thermal strengtheningeffects relative to the total range available with the present processesand methods.

In another embodiment, the thermally strengthened cover glass sheetsdescribed herein have high flatness. In various embodiments, thestrengthening system discussed herein utilizes controlled gas bearingsto support the glass material during transporting and heating, and insome embodiments, can be used to assist in controlling and/or improvingthe flatness of the cover glass sheet, resulting in a higher degree offlatness than previously obtainable, particularly for thin and/or highlystrengthened cover glass sheets. For example, sheets at least 0.6 mm canbe strengthened with improved post-strengthening flatness. The flatnessof thermally strengthened cover glass sheets embodied herein cancomprise 100 μm or less total indicator run-out (TIR) along any 50 mmlength along one of the first or second surfaces thereof, 300 μm TIR orless within a 50 mm length on one of the first or second surfaces, 200μm TIR or less, 100 μm TIR or less, or 70 μm TIR or less within a 50 mmlength on one of the first or second surfaces. In exemplary embodiments,flatness is measured along any 50 mm or less profile of the glass sheet.In contemplated embodiments, sheets with thickness disclosed herein haveflatness 200 μm TIR or less within a 20 mm length on one of the first orsecond surfaces, such as flatness 100 μm TIR or less, flatness 70 μm TIRor less, flatness 50 μm TIR or less.

According to contemplated embodiments, the strengthened cover glass orglass-ceramic articles discussed herein (e.g., cover glass sheet 500shown in FIG. 4 ) have a high-degree of dimensional consistency suchthat the thickness t thereof along a 1 cm lengthwise stretch of the body522 does not change by more than 50 μm, such as, by not more than 10 μm,not more than 5 μm, not more than 2 μm. Such dimensional consistency maynot be achievable for given thicknesses, areas, and/or magnitudes ofnegative tensile stress, as disclosed herein, by solid quenching due topractical considerations, such as cooling plate alignment and/or surfaceirregularities that may distort the dimensions.

According to contemplated embodiments, the strengthened cover glass orglass-ceramic articles discussed herein have at least one major surface(e.g., first and second surfaces 510, 520 of the strengthened coverglass or glass-ceramic sheet 500 in FIG. 4 ) that is flat such that a 1cm lengthwise profile therealong stays within 50 μm of a straight line,such as within 20 μm, 10 μm, 5 μm, 2 μm; and/or a 1 cm widthwise profiletherealong stays within 50 μm of a straight line, such as within 20 μm,10 μm, 5 μm, 2 μm. Such high flatness may not be achievable for giventhicknesses, areas, and/or magnitudes of negative tensile stress, asdisclosed herein, by liquid quenching due to practical considerations,such as warping or bending of the cover glass strengthened in theseprocesses due to convective currents and associated forces of theliquid.

Thermally Strengthened Cover Glass Sheet CTE

Another aspect comprises thermally strengthened low coefficient ofthermal expansion (CTE) cover glass sheets. As discussed above (seee.g., equations 7 and 10), thermal strengthening effects aresignificantly dependent upon the CTE of the glass of which the coverglass sheet is comprised. However, thermal strengthening of low CTEglasses may provide strengthened glass compositions having advantageousproperties, such as increased chemical resistance, or bettercompatibility with electronic devices due to low alkali content. Coverglass sheets having CTEs of 65, 60, 55, 50, 45, 40, and even 35×10⁻⁶°C.⁻¹ and below are capable of safety-glass like break patterns(“dicing”) at thicknesses of less than 4 mm, less than 3.5 mm, less than3 mm, and even at 2 mm or less. Cover glasses having CTE values of40×10⁻⁶° C.⁻¹ and below can be strengthened using the processesdescribed herein. Such low CTE glasses strengthened by the systems andmethods discussed herein can have similar surface compressions to SLGsheets strengthened by convention commercial (gas convective) processesat the same thickness. In some embodiments, the compressive stress oflow CTE glasses can comprise at least 50 MPa, at least 100 MPa, at least125 MPa, at least 150 MPa, at least 200 MPa, at least 250 MPa, at least300 MPa, or at least 400 MPa for cover glass sheets having a thicknessof no more than 1 cm, no more than 5 mm, no more than 3 mm, no more 2mm, no more than 1.5 mm, no more than 1 mm, no more than 0.75 mm, nomore than 0.5 mm, no more than 0.3 mm, no more than 0.2 mm, or no morethan 0.1 mm.

Cover glass sheets formed according to the present disclosure have amultitude of applications in consumer electronic devices, for example inelectronic devices, mobile phones, portable media players, televisions,notebook computers, watches, household appliances, tablet computerdisplays and in laminates, such as glass-interlayer-glass laminates usedin various consumer electronic devices. Stronger and thinner laminatescan be produced, resulting in weight and cost savings and fuelefficiency increases. Desirably, a thermally strengthened thin sheet maybe cold bent and laminated to a formed thicker glass, providing an easyand reliable manufacturing process not requiring any hot forming of thethin sheet.

Alpha of Thermally Tempered Cover Glass Sheet

Table IV below shows results obtained by the methods of the presentdisclosure (indicated as “Source of Method” I in the table), and afigure of merit, Alpha, that is a rough measure of the coefficient ofheat exchange obtained within the tempering process. Alpha is given by:

$\begin{matrix}{{Alpha} = \frac{CS}{\left( {t \cdot {CTE} \cdot E} \right)}} & (13)\end{matrix}$where CS is physical compressive stress (in MPa), t is thickness inmillimeters, CTE is the coefficient of thermal expansion in ° C.⁻¹, andE is the elasticity of the glass in (MPa), and yields units in ° C./mm.

TABLE IV Source Sample of Thickness CS CTE E Alpha No. Method Glass (mm)(MPa) (1/C) (MPa)** (C/mm) 1 I SLG 1.84 150 9.20E−06 68900 129 2 I SLG1.84 172 9.20E−06 68900 147 3 I SLG 1.07 190 9.20E−06 68900 230Samples 1 and 3 are repeatable values obtained from the disclosedprocesses, sample 1 using air and sample 3 using helium as the gas inthe process. Sample 2 represents a “champion” value using air within thepresent process, that is, not reliably repeatable to date. Cover glasssamples processed by the processes of the present disclosure (samples1-3) all exceeded an Alpha at 117° C./mm. Applicant believes that theslope of Alpha with thickness may have an inherent trend lower withlower glass thickness. Glass disclosed herein has an Alpha of greaterthan 20t+77, where t is thickness of the cover glass, in mm, in someembodiments.

Thermal Strengthening System and Process

In various embodiments, a process for strengthening a cover glass sheetcomprises supporting or guiding at least a portion of a cover glasssheet, such as cover glass sheet 500, into a cool or quenching zone inwhich the sheet is rapidly cooled creating a strengthened cover glasssheet having one or more of the properties discussed herein. In variousembodiments, the cover glass sheet is supported at least in part by aflow or a pressure of a gas delivered to a gap between the surfaces ofthe cover glass sheet and one or more heat sinks. In general, thetemperature of the glass sheet is above the transition temperature ofthe glass when the sheet is moved into the cool zone, and in variousembodiments, the cover glass sheet is cooled within the cooling zone bythermal conduction more than by convection. Conduction is a process ofheat transfer where energy is transmitted through interactions betweenadjacent molecules, and convection is a process of heat transfer whereenergy is communicated via motion of a fluid (e.g., air, helium, etc.),such as where heated fluid moves away from a heat source and is replacedby cooler fluid. Thus, the present system is markedly different fromconventional convection-based glass strengthening/tempering systems inwhich the primary mode of heat transfer during cooling of the glasssheet is convective.

In some embodiments, an overall process for strengthening a cover glasssheet comprises heating a cover glass sheet in a hot zone and thencooling the cover glass sheet in a cooling zone. The cover glass sheethas a transition temperature, which is the temperature at which theviscosity of the cover glass has a value of η=10¹²-10^(13.3) Poise. Thecover glass is heated sufficiently to bring the cover glass sheet abovethe transition temperature, and then moved into a cooling zone.Optionally, the cover glass can be transitioned from the hot zone to acool zone through a transition zone. In the cooling zone, the surfacesof the cover glass sheet are positioned adjacent to heat sinks, one oneither side of the cover glass sheet, each with a gap in between one ofthe cover glass surfaces and an opposing surface of the heat sink. Gasis delivered into the gaps through multiple apertures in the heat sinks,and in some embodiments, this delivered gas forms an air bearing whichsupports the cover glass between the heat sinks such that the coverglass surfaces are not in contact with the heat sinks. Within thecooling zone, the cover glass sheet is cooled by conduction more than byconvection and is cooled sufficiently to fix or create a thermallyinduced surface compression and a thermally induced central tension ofthe sheet which provides the increased strength as discussed herein. Invarious embodiments, primarily cooling via conduction is achieved byhaving a very low gap size within the cooling zone such that the coverglass sheet is close to, but not touching, the opposing surfaces of theheat sinks.

An apparatus for enabling the processes described can include a heatingzone for heating a cover glass sheet to a temperature above thetransition temperature and a cooling zone for cooling the heated coverglass sheet to provide a strengthened cover glass sheet. The apparatuscan include an optional transition zone between the heating zone and thecooling zone. The cooling zone may include a heat sink having a pair ofopposing surfaces defining a gap, within which the heated cover glasssheet is received. The cooling zone can comprise a pair of gas bearingsdisposed on opposite sides of that gap that acts to support the coverglass sheet within the gap. The gap can be configured to cool the heatedcover glass sheet by conduction more than by convection. In someembodiments, the gas bearings can include a plurality of apertures fordelivering the gas to the gap, and the gas bearing surfaces act as theheat sinks, capable of conducting heat away from the heated cover glasssheet by conduction more than by convection.

Strengthening processes and equipment disclosed herein (see generallyFIGS. 21-25 ) allow for strengthening of cover glass or glass-ceramicarticles (see generally FIGS. 4-7 and 27-30 ) by an inventive form ofthermal tempering. The processes allow for steep, tensile stress versusthickness/depth curves (see generally FIG. 6 ), particularly steep inslope near the surface of the cover glass or glass-ceramic articles,which enable strengthening of the cover glass or glass ceramic articlesto particularly high levels of negative tensile stress for a giventhickness near the surface of the respective articles, without requiringstrengthening by ion-exchange or laminating different glasses. However,in some embodiments, the thermal tempering processes disclosed hereinmay be augmented with ion-exchange or applied to glass-to-glasslaminations. The thermal tempering processes disclosed herein enableparticularly high levels of strengthening in large-area articles (e.g.,sheets) that may be too large for strengthening via conventional thermaltempering methods, such as due to alignment limitations of contactquench equipment, cooling rate limitations of conventional convectionsystems, and/or warping damage associated with liquid quench tempering.The processes disclosed herein uniquely allow high levels ofstrengthening in particularly thin sheets that may be too thin forstrengthening via conventional tempering methods, such as duesensitivity to breakage or fracture of the thin cover glass or glassceramic articles during the strengthening process and associated contactforces with solid or liquid quenching and/or due to the cooling ratelimitations of conventional convection tempering. However, in othercontemplated embodiments, cover glass or glass ceramic articles asdisclosed herein may be manufactured with at least some solid or liquidquenching, such as in combination with the unique strengtheningprocesses disclosed herein.

One embodiment of a method according to this disclosure is illustratedin the flow chart of FIG. 16 . The method or process 100 includes a step140 of providing a cover glass sheet that is at a temperature above atransition temperature of the glass sheet. The method or process 100also includes the step 160 of supporting a cover glass sheet at least inpart by a gas (through gas flow and pressure). Step 160 includes, whilethe cover glass sheet is support by the gas, cooling the sheet: 1) byconduction more than by convection through the gas to a heat sink, and2) sufficiently to create or fix a thermally-induced surface compressionstress and a thermally-induced central tension stress, of the sheet whenat ambient temperature.

According to a variation on the embodiment of FIG. 16 , depicted asmethod 100′ in the flow chart of FIG. 17 , the method can include thestep 110 of heating a cover glass sheet sufficiently such that the sheetis above a transition temperature of the glass. As part of, or aspreparation for, the cooling step 160, the method 100′ furthercomprises, in step 120, providing a heat sink (whether as a single pieceor in separate pieces) having first and second heat sink surfaces (seegenerally FIGS. 21-25 ), each having apertures therein. In step 130A themethod further includes positioning a first sheet surface facing a firstheat sink surface across a first gap and, in step 130B, positioning thesecond sheet surface facing a second heat sink surface across a secondgap. The heat sink surfaces can include apertures and/or can be porous.The method 100′ can further include, in step 160, cooling the sheet, byconduction more than by convection through a gas to the respective heatsink surfaces, sufficiently to strengthen the cover glass (e.g., tosufficiently create or fix in the sheet a thermally-induced surfacecompression stress and a thermally-induced central tension stress). Thestep 160 also can include delivering the gas to the first and secondgaps through the apertures or porous heat sink, and in some suchembodiments, the gas is delivered to form air bearings that support thecover glass sheet adjacent the heat sinks. In some embodiments, the gasis delivered only through the apertures of the heat sink or only throughthe pores or pores and apertures of the porous heat sink.

These and other related methods of this disclosure go against thecurrently dominant technique of gas-convection-cooling by usingconduction as the dominant mode of cooling, instead of convection.Instead of a solid-to-gas (glass to air) heat exchange, methodsdescribed herein use a solid-to-solid (glass to heat sink) heatexchange, mediated across a small gap by a small amount of gas (e.g.,without physical contact between glass surfaces and heat sink), both tobegin and to complete the cooling that produces thermal strengthening.Although some convection is present as gas (e.g., air bearing gas) flowsinto the small gap, conduction directly across the gap through the gasand into the heat sink is the principal mode of cooling. Applicant hasdetermined that dominance of conductive heat transfer increases the rateof heat transfer relative to convection dominant cooling processes.

Because solid-to-solid conduction (even across the gap) allows for morerapid heat flow than convection, the cooling rate increases needed forthinner cover glass sheets are not tied to gas velocity and volume.According to various embodiments, without the constraints typicallyimposed by gas flow and gap size in a convective system, gas flow andgap size can be selected, controlled or optimized for other purposes,such as for controlling stiffness of the gas cushion in the gap, forsupporting the sheet, for flattening or otherwise shaping a sheet, foroptimizing heat conduction, for maintaining sheet flatness and/or shapeduring thermal strengthening, and/or for balancing ease of sheethandling with high cooling rates. For example, in some embodiments,because cooling is not via convection, helium becomes an economicallyviable alternative to air in the system of the present disclosure due tothe very low gas flow rates that support the gas bearing, and in suchembodiments, helium offers thermal conductivity about five times that ofair. Even helium with prices assumed at multiples of those availabletoday becomes an economically viable alternative at the low flow ratesof the system of the present disclosure.

Further, because the system of the present disclosure decreases thevolume of air flowing over a cover glass sheet during cooling (relativeto convective systems), the systems and methods discussed hereindecrease the potential risk of deformation of hot thin sheets of coverglass typically caused by the high speed, high volume air flows neededin conventional convection based tempering systems. This also allowssofter, higher temperature cover glass sheets to be handled with no orminimal distortion, further improving the achievable degree ofstrengthening. Eliminating high air flow rates also eases problemssometimes seen in transporting the sheet into the quenching chamber(moving against the high air flow) and in keeping the high-flow, coolerair from entering into and cooling the adjacent parts of the furnaceused to heat the sheet.

Further the use of conduction, through a gas, may mitigate contactdamage, warping, shaping, etc. associated with conventional liquidcontact or solid contact quench tempering. Use of a gas as anintermediate conductor preserves the surface quality of the processedarticles by avoiding solid-to-solid contact. Mediating the highconduction rates through a gas also avoids liquid contact. Some types ofliquid quenching can introduce unwanted distortions, spatial variationin tempering and contamination of the cover glass surfaces. Theseembodiments essentially provide non-contact (except by a gas) but veryhigh-rate cooling. In other embodiments, as discussed above, solid- orliquid-contact may be included.

Power Consumption of Thermal Tempering System/Process

Another advantage of avoiding high air flow rates lies in the power andenergy savings achieved by using solid-gas-solid conduction as theprimary cover glass cooling mechanism. Points A and B of FIG. 18 andFIG. 19 represent a high-end estimate of peak power use of the airbearing, per square meter of cover glass sheet, by a compressed airsupply at relatively high flow. Practical low-end peak power use ofcompressed air could be as little as 1/16 of the values shown. Points Aand B do not include active cooling of the heat sink, however, which canbe included in some embodiments, especially where a machine is incontinuous, quasi-continuous or high frequency operation.

Referring again to FIG. 18 and FIG. 19 , points A′ and B′ represent theconservatively estimated peak power levels for operation of the airbearing at points A and B when active cooling of the heat sink surfacesis factored in, assuming the thermal load equivalent of a 300° C. dropin cover glass sheet temperature is accomplished by an active coolingsystem having a thermal-to-mechanical (or electrical) efficiency ratioof 7.5 to 1, within a time limit of 2.1 seconds for point A′ and within1 second for point B′. (These points correspond approximately to coverglass sheets actually tempered in the apparatus described herein.)

Although the four points within region R of FIG. 18 and FIG. 19illustrate the significance of the improvement obtainable by the methodsand systems of the present disclosure (at least to some degree), itshould be noted that the full benefits are likely significantlyunderstated in the figures because power demand is the quantityrepresented. For example, peak power of air blowers, as represented bythe curve N, is not efficiently turned on and off, typically requiringgated airways to block off large fans, which still rotate (but atreduced load), when air is not needed. Peak power demands of fluidcooling systems such as chilled water plants, represented by the pointsA′ and B′ as examples easily achievable according to the presentdisclosure, can generally be much more efficiently accommodated, andeffective peak power would be significantly lower, approaching A′ and B′only as fully continuous operation is approached. Thus, the differencein total energy demands would tend to be greater than the difference forpeak power demand, which is represented in the figure. In someembodiments, the processes described herein have peak powers of lessthan 120 KW/m², less than 100 KW/m², less than 80 KW/m² to thermallystrengthen a cover glass sheet of 2 mm thickness or less.

Heat Transfer from Thin Cover Glass Sheet During Thermal Tempering

In general, heat transfer from the thin cover glass sheet in the systemand process of the present disclosure includes a conduction component, aconvection component and a radiant component. As noted above andexplained in detail herein, the thermal tempering system of the presentdisclosure provides for thin cover glass tempering by utilizingconductive heat transfer as the primary mechanism for quenching the thincover glass sheets.

The following is Applicant's understanding of the underlying theory. Itmay well occur to one of ordinary skill in the art of glass tempering,in which conduction effects are normally so small as to be commonlyignored in favor of analysis of convection and radiation alone, to askwhether sufficiently high cooling rates for thin cover glass sheets(such as at 2 millimeters and below) are actually achievable byconduction through a gas such as air—and if so, whether such rates areachievable at practical gap sizes.

The amount of thermal conduction at conditions embodied in processesusing systems described herein can be determined via the following.First, in the context of thermal strengthening by conduction as in thepresent disclosure, the thermal conductivity of the gas within the gapmust be evaluated in the direction of conduction, which is along athermal slope. Air at high temperature, at or near the surface of thesheet being cooled, has significantly higher thermal conductivity thanair at a lower temperature, such as air at or near room temperature ator near the surface of the heat sink (the nominal thermal conductivityof (dry) room temperature air (25° C.) is approximately 0.026 W/m·K). Anapproximation that assumes air over the whole gap to be at the averagetemperature of the two facing surfaces at the start of cooling is used.At the start of cooling, a cover glass sheet may be at a temperature of670° C., for example, while the heat sink surface may start at 30° C.,for example. Accordingly, the average temperature of the air in the gapwould be 350° C., at which dry air has a thermal conductivity of about0.047 W/m·K; more than 75% higher than its thermal conductivity at roomtemperature and sufficiently high to conduct large amounts of heatenergy through gaps of the sizes within the system of the presentdisclosure, as discussed below, assuming the sheet is finished to areasonably high degree of surface and thickness consistency.

To illustrate, Q_(cond), the conductive component of the rate of heattransfer through a gap of distance g which gap has an area A_(g) (in adirection everywhere perpendicular to the direction of the gap distanceg) may be given by:

$\begin{matrix}{Q_{cond} = \frac{A_{g}{k\left( {T_{S} - T_{HS}} \right)}}{g}} & (14)\end{matrix}$where k is the thermal conductivity of the material (gas) in the gapevaluated in the direction of (or opposite of) heat conduction, T_(S) isthe temperature of the cover glass surface and T_(HS) is the temperatureof the heat sink surface (or the heat source surface, for otherembodiments). As mentioned above, to evaluate k rigorously would requireintegrating the thermal conductivity of the gas along (or against) thedirection of conductive heat flow, as the thermal conductivity of thegas varies with temperature—but as a good approximation, k may be takenas the value of k for the gas in the gap when at the average of thetemperatures of the two surfaces, T_(S) and T_(HS).

Reframing equation (14) in units of heat transfer coefficient (units ofheat flow power per meter squared per degree Kelvin) gives:

$\begin{matrix}{\frac{Q_{cond}}{A_{g}\left( {T_{S} - T_{HS}} \right)} = \frac{k}{g}} & (15)\end{matrix}$so the effective heat transfer coefficient for conduction across the gapis the thermal conductivity of the medium in the gap (air in this case)(in units of W/mK) divided by the length of the gap (in meters), givinga value of Watts per meter squared per degree of temperature difference.Table V shows the heat transfer coefficients (k/g), due to conductionalone, for air and helium filled gaps of gap sizes from 10 μm up to 200μm in steps of 10 μm each.

TABLE V Air Helium conductivity (W/m/K) 0.047 conductivity (W/m/K) 0.253heat trans coeff. heat trans coeff. Gap (m) W/m²/K cal/s/cm² Gap (m)W/m²/K cal/s/cm² 0.00001 4700 0.11226 0.00001 25300 0.604291 0.000022350 0.05613 0.00002 12650 0.302145 0.00003 1566.67 0.03742 0.000038433.33 0.20143 0.00004 1175 0.028065 0.00004 6325 0.151073 0.00005 9400.022452 0.00005 5060 0.120858 0.00006 783.333 0.01871 0.00006 4216.670.100715 0.00007 671.429 0.016037 0.00007 3614.29 0.086327 0.00008 587.50.014032 0.00008 3162.5 0.075536 0.00009 522.222 0.012473 0.000092811.11 0.067143 0.0001 470 0.011226 0.0001 2530 0.060429 0.00011427.273 0.010205 0.00011 2300 0.054936 0.00012 391.667 0.009355 0.000122108.33 0.050358 0.00013 361.538 0.008635 0.00013 1946.15 0.0464840.00014 335.714 0.008019 0.00014 1807.14 0.043164 0.00015 313.3330.007484 0.00015 1686.67 0.040286 0.00016 293.75 0.007016 0.000161581.25 0.037768 0.00017 276.471 0.006604 0.00017 1488.24 0.0355470.00018 261.111 0.006237 0.00018 1405.56 0.033572 0.00019 247.3680.005908 0.00019 1331.58 0.031805 0.0002 235 0.005613 0.0002 12650.030215

FIG. 20 (Prior Art) shows an industry-standard curve from about 35 yearsago (with reference line at 2 mm added) showing the heat transfercoefficient required to fully temper a sheet of glass, as a function ofthickness in mm, under certain assumed conditions. As may be seen from acomparison of Table V with FIG. 20 , an air-filled gap of approximately40 μm can allow full tempering of 2 mm thick cover glass by conduction.While slightly less than 40 micrometers is a rather small gap, planarporous air bearings in conveyor applications may generally be reliablyrun with gaps of as low as 20 micrometers. Thus 37 micrometers isachievable for an air gap fed by pores in the heat sink surface. Usinghelium (or hydrogen, with similar thermal conductivity) as the gas, agap of about 200 μm can be used to fully temper 2 mm thick cover glass.Using helium or hydrogen as the gas allows for a gap size about 5 timeslarger for the same heat transfer coefficient. In other words, usinghelium or hydrogen as the gas in the gap increases the heat transfercoefficient available for quenching by about 5 times at the same gapsize. So even with air the spacing is not impractical, and with highconductivity gases, the gap spacing is relatively easy to achieve, evenfor sheet thicknesses smaller than 2 millimeters.

In addition to cooling through a gas by conduction more than byconvection, another embodiment includes heating (or heating and/orcooling) through a gas by conduction more than by convection. Regardingthe relative contributions of conduction and convection, whether forheating or cooling, the convective Q_(conv), component of the rate ofheat transfer across the gap (or gaps) may be given by:

$\begin{matrix}{Q_{conv} = {e\overset{.}{m}{C_{p}\left( {\frac{T_{S} + T_{HS}}{2} - T_{i}} \right)}}} & (16)\end{matrix}$where {dot over (m)} is the mass flow rate of the gas, Cp is thespecific heat capacity of the gas, T_(i) is the inlet temperature of thegas as it flows into the gap, and e is the effectiveness of the heatexchange between the gas flowing in the gap, the sheet surface and thesurface of the heat sink/source (the “walls” of the gap). The value of evaries from 0 (representing zero surface-to-gas heat exchange) to 1(representing the gas fully reaching the temperature of the surfaces).The value of e can be computed by those skilled in the art of heattransfer using, for example, the e-NTU method.

Typically, however, if the gap between the surface of the sheet and thesurface of the heat sink/source is small, the value of e will be verynearly equal to 1, meaning the gas heats nearly completely—to equal, onaverage, the average of the temperatures of the two surfaces on eitherside—before it leaves the gap. Assuming e=1 (a slight overestimate ofthe rate of convective heat transfer), and the gas being supplied to thegap through the surface of the heat sink/source, it can be assumed thatthe initial temperature of the gas in the gap is the same as thetemperature of the surface of the heat sink/source (T_(i)=T_(HS)). Therate of heat transfer due to convection may then be simplified to:

$\begin{matrix}{Q_{conv} = {\overset{.}{m}{C_{p}\left( \frac{T_{S} - T_{HS}}{2} \right)}}} & (17)\end{matrix}$

At the temperatures typically useful for heat strengthening or heattreating of glass and similar materials, radiative heat transfer out ofthe sheet under treatment is relatively small. To cool (or heat,assuming the amount of radiation from the heat source when heating isnot too high) the sheet (e.g., sheet 200 shown in FIG. 21 ) principallyby conduction, in the area of the gap (e.g., gaps 204 a, 204 b shown inFIG. 21 ), thus requires only that:Q _(cond) >Q _(conv)  (18)Combining (18) with equations (14) and (17) gives the followingconditional:

$\begin{matrix}{\frac{k}{g} > \frac{\overset{.}{m}C_{p}}{2A_{g}}} & (19)\end{matrix}$which, when held, will essentially ensure that the sheet, in the area ofthe gap at issue, is cooled (or heated) principally by conduction.Accordingly, the mass flow rate {dot over (m)} of the gas should be lessthan 2 kA_(g)/gC_(p), or 2 k/gC_(p) per square meter of gap area. In anembodiment, {dot over (m)}<B·(2 kA_(g)/gC_(p)), where B is the ratio ofconvective cooling to conductive cooling. As used herein, B is apositive constant less than one and greater than zero, specificallyhaving a value of ⅔ or less, or even ⅘ or 9/10 or less. Generally, mshould be kept as low as possible, consistent with the needs of usingthe gas flow to control the position of the cover glass sheet (e.g.,sheet 200 shown in FIG. 21 relative to the heat sink surface(s)) (e.g.,heat sink surfaces 201 b, 202 b, shown in FIG. 21 ) or the position ofthe heat exchange surfaces themselves. The ratio of convective coolingto conductive cooling can be any value from less than one to 1×10⁻⁸. Insome embodiments, B is less than 0.9, 0.8, 0.7, 0.6, 0.5, 0.4, 0.1,5×10⁻², 1×10⁻², 5×10⁻³, 1×10⁻³, 5×10⁻⁴, 1×10⁻⁴, 5×10⁻⁵, 1×10⁻⁵, 5×10⁻⁶,1×10⁻⁶, 5×10⁻⁷, 1×10⁻⁷, 5×10⁻⁸, or 1×10⁻⁸. In some embodiments, m isminimized, consistent with the needs of using the gas flow to supportand control the sheet position relative to the heat sink surface(s). Inother embodiments, m should be selected to control the position of theheat exchange surfaces themselves, relative to the sheet.

In various embodiments, the mass flow rate m of the gas within theconductive-based cooling system of the present disclosure issubstantially lower as compared to the conventional convection-basedtempering systems. This substantially lower gas flow rate allows theconductive system to be operated at substantially reduced power usage,as discussed herein. Further, in at least some embodiments, the reducedgas flow rate also results in a substantially quieter cooling system ascompared to a conventional convective cooling system. In suchembodiments, the decrease in noise may increase operator safety byreducing the potential for hearing damage and even reducing oreliminating the need for operators to use hearing protection.

As will be understood, in embodiments in which a sheet of cover glassmaterial is supported on air bearings between opposing heat sinksurfaces, conductive heat transfer will occur from both sides of thecover glass sheet to both heat sink surfaces Thus, in such embodiments,the cover glass sheet has first and second sheet surfaces, and coolingof the cover glass sheet is performed by positioning the first sheetsurface (e.g., a lower surface of the cover glass sheet) adjacent to afirst heat sink surface (e.g., a surface of a lower heat sink) such thata first gap is located between the first sheet surface and the firstheat sink surface and by positioning the second sheet surface (e.g., anupper surface of the cover glass sheet) adjacent to a second heat sinksurface (e.g., a surface of an upper heat sink) such that a second gapis located between the second sheet surface and the second heat sinksurface. In such embodiments, thermal conduction from the first sheetsurface to the first heat sink surface and from the second sheet surfaceto the second heat sink surface is permitted to occur. In suchembodiments, the first gap has a length across the first gap of g₁ andan area of the first gap of A_(g1), and the second gap has a lengthacross the second gap of g₂ and an area of the second gap of A_(g2). Insuch embodiments, a first flow of a first gas to the first gap isprovided, and a second flow of a second gas to the second gap isprovided. As will be understood, similar to the discussion above, thefirst gas has a heat capacity C_(p1) and a thermal conductivity k₁, andthe first flow is provided at a mass flow rate {dot over (m)}₁. In suchembodiments, {dot over (m)}₁ is greater than zero and less than (2k₁A_(g1))/(g₁C_(p1)). Further, the second gas has a heat capacity C_(p2)and a thermal conductivity k₂, and the second flow is provided at a massflow rate {dot over (m)}₂. In such embodiments, {dot over (m)}₂ isgreater than zero and less than (2 k₂A_(g2))/(g₂C_(p2)). In suchembodiments, the first and second flows contact the cover glass sheetsuch that the cover glass sheet is supported without touching the heatsink surfaces. In this manner, the sheet is cooled by conduction morethan by convection in a manner to create a surface compressive stressand a central tension of the sheet.

Cover Glass Strengthening System Including High Conduction Cooling Zone

Referring to FIG. 21 , a diagrammatic cross-section of a high conductionglass cooling/quenching station and of a glass sheet being cooled byconduction more than by convection is shown. A hot glass sheet 200 hasits first and second (major) surfaces 200 a, 200 b each facing arespective first and second surface 201 b, 202 b of respective first andsecond heat sinks 201 a, 202 a across respective gaps 204 a and 204 b.Gas 230 is fed through the first and second surfaces 201 b, 202 b asrepresented by the arrows, to supply the gaps 204 a, 204 b, and toassist in keeping the cover glass sheet centered or otherwise positionedbetween the heat sinks 201 a, 202 a. The air or other gas may leavepassing by the edges of the heat sinks 201 a, 202 a as shown by arrows240. By choosing the size of the gaps 204 a, 204 b and the gas and theflow rate of the gas 230 in accordance with the discussion herein, thecover glass sheet 200 will be cooled more by conduction than convection.In specific embodiments, cover glass sheet 200 is cooled by heat sinks201 a and 202 a such that more than 20%, specifically more than 50%, andmore specifically more than 80%, of the thermal energy leaving a heatedarticle, such as cover glass sheet 200, crosses the gaps, such as gaps204 a and 204 b, and is received by the heat sink 201 a and 202 a.

In some embodiments, the gaps 204 a, 204 b are configured to have athickness or distance across the gap sufficient such that the heatedcover glass sheet is cooled by conduction more than by convention. Aswill be understood, size of gaps 204 a, 204 b generally is the distancebetween the major cover glass surfaces and the opposing heat sinksurfaces.

In some embodiments, gaps 204 a and 204 b may have a thicknesses ofabout (e.g., plus or minus 1%) 100 μm or greater (e.g., in the rangesfrom about 100 μm to about 200 μm, from about 100 μm to about 190 μm,from about 100 μm to about 180 μm, from about 100 μm to about 170 μm,from about 100 μm to about 160 μm, from about 100 μm to about 150 μm,from about 110 μm to about 200 μm, from about 120 μm to about 200 μm,from about 130 μm to about 200 μm, or from about 140 μm to about 200μm). In other embodiments, gaps 204 a and 204 b may have a thicknessesof about (e.g., plus or minus 1%) 100 μm or less (e.g., in the rangesfrom about 10 μm to about 100 μm, from about 20 μm to about 100 μm, fromabout 30 μm to about 100 μm, from about 40 μm to about 100 μm, fromabout 10 μm to about 90 μm, from about 10 μm to about 80 μm, from about10 μm to about 70 μm, from about 10 μm to about 60 μm, or from about 10μm to about 50 μm).

Heat sinks 201 a, 202 a may be solid or porous configurations. Suitablematerials include, but are not limited to, aluminum, bronze, carbon orgraphite, stainless steel, etc. Heat sink dimensions may be designed tobe sufficient to address the size of the cover glass sheet and toefficiently and effectively transfer heat without changing the heat sinktemperature significantly. In the case where heat sinks 201 a and/or 202a are porous, they may still include additional apertures or holes forflowing gas or may use the porous structure to provide flow, or both. Insome embodiments, the heat sinks further comprise passages to allowfluid flow for controlling the temperature of the heat sink, describedin more detail in FIGS. 23-25 and below.

Eliminating high gas flow rates of the prior art may enable use of verysmall apertures or pores 206, as shown in FIG. 21 , in the heat sinkface to provide the gas to the gap(s). In some embodiments, aperturesmay be less than 2 mm, less than 1.5 mm, less than 1 mm, less than 0.5mm, less than 0.25 mm, or less than or equal to 200, 150, 100, 50, 30,20, or 10 μm, when measured in the smallest direction (e.g., diameter inthe case of circular apertures). In some embodiments, the apertures arefrom about (e.g., plus or minus 1%) 10 μm to about 1 mm, about 20 μm toabout 1 mm, or about 50 μm to about 1 mm.

Spacing between adjacent apertures 206 can be from about (e.g., plus orminus 1%) 10 μm to about 3 mm, about 20 μm to about 2 mm, or about 50 μmto about 1 mm, measured edge-to-edge of apertures. Small apertures orpores may function as individual flow restrictors, providinghigh-performance, gas-bearing-type dynamics, such as high levels ofstiffness and consistency of support of the sheet to position the sheetand control gap size, allowing for high homogeneity of thermalstrengthening effects to avoid or reduce stress birefringence. Further,because very small pores or apertures may be used, the relative amountof solid matter at the surface of the heat sink facing the sheet surfaceacross the gap(s) can be maximized, thereby increasing conductive heatflow.

According to various embodiments, use of such apertures 206 as the onlypath for providing gas to the gaps 204 a, 204 b, and desirably usingapertures 206 that lie in directions close to normal to the heat sinksurface 201 b, 202 b, ensures that air-bearing type dynamics areoptimized, and not compromised by gas flows from larger apertures, orfrom sources other than through the heat sink surface(s) 201 b, 202 badjacent to the sheet 200, or by other excessive lateral flow. In otherembodiments gas may be provided to the gaps 204 a, 204 b via othersources, such as in addition to the apertures 206 or pores. Accordingly,aspects of the present disclosure allow for power and energy savings byuse of low gas flows and solid-gas-solid conduction, such as relative toconventional convective tempering processes.

FIGS. 22-25 show an exemplary embodiment of a cover glass strengtheningsystem 300 according to this disclosure. FIG. 22 shows a schematiccross-sectional diagram of the system 300, in which a cover glass sheetcan be cooled via conduction of heat from the cover glass sheet, througha gas into a conductive heat sink. The apparatus includes a hot zone310, a cold zone 330, and a transition gas bearing 320. Transition gasbearing 320 moves or directs a cover glass article (e.g., cover glasssheet 400 a) from the hot zone 310 to the cold zone 330 such that nocontact or substantially no contact occurs between the cover glass andthe bearings. The hot zone 310 has gas bearings 312 each fed from a hotzone plenum 318, and the bearings 312 have cartridge heaters 314inserted into holes through the bearings 312, which serve to heat thehot zone gas bearings 312 to a desired starting process temperature. Acover glass sheet (hot zone) 400 a is kept between the hot zone gasbearings 312 for a duration long enough to bring it to a desiredpre-cooling temperature (e.g., above the transition temperature).

In some embodiments, heating the sheet in the hot zone may be donepredominantly via conduction of heat from a heat sink through a thin gasbarrier. The conductive heating processes used in the hot zone can besimilar to, but the reverse of the cooling processes described herein(e.g., pushing heat into the cover glass sheet).

In some embodiments, gaps 316, between the hot zone gas bearings 312 andthe cover glass sheet 400 a, may be relatively large, on the order of0.05″ (1.27 mm) to 0.125″ (3.175 mm) or greater, since the cover glasssheet 400 a may be heated up relatively slowly and thermal radiationfrom the hot gas bearings 312 into the cover glass sheet 400 a isadequate for this purpose. In other embodiments, hot zone gap size maybe as small as 150 microns per side or 500 microns per side. Smallergaps may be advantageous, in some embodiments, because they enable thebearings to have better “stiffness”—i.e., ability to centralize thecover glass and therefore flatten it while it is in its softened state.In some embodiments, the process may re-form the cover glasssheets—flattening them—in the initial heating step, for example via thepressure supplied by the gas bearings 312. In some embodiments, the topand bottom hot zone bearings may be on actuators, allowing for changingthe gap width in a continuous manner or, alternatively, allowing thecover glass to be brought into the hot zone when the gap is large andthen compressing the gap to flatten the cover glass while it is stillsoft.

Process temperatures are dependent on a number of factors, includingcover glass composition, cover glass thickness, cover glass properties(CTE, etc.), and desired level of strengthening. Generally, the startingprocess temperature may be any value between the cover glass transitiontemperature and the Littleton softening point, or in some embodiments,even higher. For SLG, for example, system 300 heats the cover glasssheet 400 a to a temperature between about (e.g., plus or minus 1%) 640to about 730° C. or between about 690 to about 730° C. In someembodiments, system 300 heats the glass sheet 400 a to a temperaturefrom about (e.g., plus or minus 1%) 620 to about 800° C., about 640 toabout 770° C., about 660 to about 750° C., about 680 to about 750° C.,about 690 to about 740° C., or about 690 to about 730° C.

The cover glass sheet 400 a is heated to its desired starting processtemperature (e.g., above the cover glass transition temperature), and itis then moved from the hot zone 310 to the cold zone 330 using anysuitable means. In some embodiments, moving the cover glass sheet 400 afrom the hot zone 310 to the cold zone 330 may be accomplished by, forexample (1) tilting the entire assembly such that gravity acting on thecover glass sheet forces it to move to the cold zone, (2) blocking offthe gas flow from the leftmost exit of the hot zone 310 (the sides areenclosed in this embodiment), thereby forcing all of the gas emanatingfrom all of the gas bearings to exit from the rightmost exit of the coldzone, causing fluid forces to be exerted on the cover glass sheet 400 aand causing it to move to the cold zone 330, or (3) by a combination of(1) and (2))

The transition gas bearings 320 may be supplied with gas by transitionbearing plenums 328. The solid material thickness behind the surfaces ofthe transition gas bearings 320 may be thin, of low thermal mass and/orlow thermal conductivity, allowing for reduced heat conduction from thehot zone 310 to the cold zone 330. The transition gas bearings 320 mayserve as a thermal break or transition between the two zones 310 and 330and may serve to transition from the larger gaps 316 of the hot zonedown to small gaps 336 of the cold zone 330. Further, the low thermalmass and/or low thermal conductivity of transition gas bearings 320limit(s) the amount of heat transfer and therefore cooling experiencedby cover glass sheet 400 a while passing past transition gas bearings320.

Once the cover glass sheet (cold zone) 400 b moves into the cold zone330 and into the channel 330 a, it is stopped from exiting the rightside exit by a mechanical stop or any other suitable blocking mechanism,shown as stop gate 341. Once the consumer electronic glass or coverglass sheet 400 b cools sufficiently that the center has passed theglass transition (in the case, for example, of 1 mm thick SLG, to belowabout 490° C., corresponding in this example to about 325° C. at thesurface), the stop gate 341 may be moved, unblocking cold zone channel330 a, and then the cover glass sheet 400 b may be removed from thesystem 300. If desired, the cover glass sheet 400 b may be left in thecold zone 330 until somewhere near room temperature before removal.

As noted above, within hot zone 310, cover glass sheet 400 is heated toa temperature above the cover glass transition temperature of the coverglass sheet. In the embodiment shown in FIG. 22 , the cold zone 330includes a channel 330 a for receiving heated cover glass sheet 400 bthrough an opening 330 b, conveying the cover glass sheet 400 b, andcooling the cover glass sheet 400 b in the cold zone. In one or moreembodiments, the channel 330 a includes a conveyance system that mayinclude gas bearings, roller wheels, conveyor belt, or other means tophysically transport the cover glass sheet through the cold zone. Asshown in FIG. 22 , cold zone 330 includes gas bearings 332 which are fedplenums 338 that are separate from hot zone plenums 318 and transitionplenums 328.

As shown in FIG. 22 , the cold zone 330 includes one or more heat sinks331 disposed adjacent to the channel 330 a. Where two heat sinks areutilized, such heat sinks may be disposed on opposite sides of thechannel 330 a, facing each other across a channel gap 330 a. In someembodiments, the heat sinks include a plurality of apertures 331 a whichform part of the gas bearing 332, and the surfaces of the cold gasbearings 332 of the cold zone 330 serve as the two heat sink surfaces.Due to the low air flow rate within channel 330 a and the small size ofchannel gap 330 a, cover glass sheet 400 b is cooled within cold zone330 primarily by conduction of heat from the cover glass sheet acrossthe gap and into the solid heat sinks 331, without the cover glass sheet400 b touching the heat sink surfaces.

In some embodiments, the heat sinks and/or the surfaces thereof may besegmented. As noted above, in some embodiments, the heat sinks may beporous, and in such embodiments, the apertures through which the gas forgas bearings 332 is delivered are the pores of the porous heat sinks.The plurality of apertures 332 b, a gas source and the channel gap 330 amay be in fluid communication. In some embodiments, the gas flowsthrough the apertures 331 a to form gas cushions, layers or bearings inthe channel gap 330 a. The gas cushions of some embodiments prevent thecover glass sheet 400 b from contacting the heat sink 331 surfaces. Thegas also serves as the gas through which the cover glass sheet 400 b iscooled by conduction more than by convection.

Because cooling occurs essentially by solid-to-solid heat conductionacross the gaps, issues not present in convection-dominated cooling mayneed to be addressed. For example, for tempering of a large, thin sheet,the sheet may be (1) introduced quickly into the cold zone, optionallyat higher speeds than those typically used in convection-based quenchingand/or (2) the process is operated in a quasi-continuous mode, in whichmultiple sheets are heated and cooled one after the other in acontinuous stream with little space between them, and where the heatsink is actively cooled such that it reaches a thermal equilibrium sothat the front and trailing edges of the large sheets have similarthermal history.

In some embodiments, the gas flowed through the apertures 331 a coolsthe heat sinks. In some embodiments, the gas flowed through theapertures both facilitates heat conduction, from the cover glass, acrossthe gap, into the heat sinks, and also cools the heat sinks 331. In someinstances, a separate gas or fluid may be used to cool the heat sinks331. For instance, the heat sinks 331 may include passages 334, forflowing a cooling fluid therethrough to cool the heat sinks 331, as ismore fully described with respect to FIG. 23 . The passages 334 can beenclosed.

Where two heat sinks are used (i.e., a first heat sink and the secondheat sink), one or more gas sources may be used to provide a gas to thechannel gap 330 a. The gas sources may include the same gas as oneanother or different gases. The channel gap 330 a may, therefore,include one gas, a mixture of gases from different gas sources, or thesame gas source. Exemplary gases include air, nitrogen, carbon dioxide,helium or other noble gases, hydrogen and various combinations thereof.The gas may be described by its thermal conductivity when it enters thechannel 330 a just before it begins to conductively cool the cover glasssheet 400 b. In some instances, the gas may have a thermal conductivityof about (e.g., plus or minus 1%) 0.02 W/(m·K) or greater, about 0.025W/(m·K) or greater, about 0.03 W/(m·K) or greater, about 0.035 W/(m·K)or greater, about 0.04 W/(m·K) or greater, about 0.045 W/(m·K) orgreater, about 0.05 W/(m·K) or greater, about 0.06 W/(m·K) or greater,about 0.07 W/(m·K) or greater, about 0.08 W/(m·K) or greater, about 0.09W/(m·K) or greater, about 0.1 W/(m·K) or greater, about 0.15 W/(m·K) orgreater, or about 0.2 W/(m·K) or greater).

The processes and systems described herein allow for high heat transferrates which, as discussed above, allow for a strengthening degree oftemperature differential to form within even a very thin cover glasssheet. Using air as the gas, with gaps between the cover glass sheet andthe heat sinks, heat transfer rates as high as 350, 450, 550, 650, 750,1000, and 1200 kW/m² or more are possible through conduction alone.Using helium or hydrogen, heat transfer rates of 5000 kW/m² or more canbe achieved.

The heat sinks 331 of one or more embodiments may be stationary or maybe movable to modify the thickness of the channel gap 330 a. Thethickness of the cover glass sheet 400 b may be in a range from about0.4 times the thickness to about 0.6 times the thickness of channel gap300 a, which is defined as the distance between the opposing surfaces ofthe heat sinks 331 (e.g., upper and lower surface of heat sinks 331 inthe arrangement of FIG. 22 ). In some instances, the channel gap isconfigured to have a thickness sufficient such that the heated coverglass sheet is cooled by conduction more than by convection.

In some embodiments, the channel gap may have a thickness such that whencover glass sheet 400 b is being conveyed through or located within thechannel 330 a, the distance between the major surfaces of the coverglass sheet 400 b and the heat sink surface (e.g., the gap sizediscussed above) is about (e.g., plus or minus 1%) 100 μm or greater(e.g., in the range from about 100 μm to about 200 μm, from about 100 μmto about 190 μm, from about 100 μm to about 180 μm, from about 100 μm toabout 170 μm, from about 100 μm to about 160 μm, from about 100 μm toabout 150 μm, from about 110 μm to about 200 μm, from about 120 μm toabout 200 μm, from about 130 μm to about 200 μm, or from about 140 μm toabout 200 μm). In some embodiments, the channel gap may have a thicknesssuch that when cover glass sheet 400 b is being conveyed through thechannel, the distance between the cover glass sheet and the heat sinksurface (the gap or gaps 336) is about (e.g., plus or minus 1%) 100 μmor less (e.g., in the range from about 10 μm to about 100 μm, from about20 μm to about 100 μm, from about 30 μm to about 100 μm, from about 40μm to about 100 μm, from about 10 μm to about 90 μm, from about 10 μm toabout 80 μm, from about 10 μm to about 70 μm, from about 10 μm to about60 μm, or from about 10 μm to about 50 μm). The total thickness of thechannel gap 330 a is dependent on the thickness of the cover glass sheet400 b, but can be generally characterized as 2 times the distancebetween the heat sink surface and the cover glass sheet, plus thethickness of the cover glass sheet. In some embodiments, the distance orgaps 336 between the cover glass sheet and the heat sinks may not beequal. In such embodiments, the total thickness of the channel gap 330 amay be characterized as the sum of the distances between the cover glasssheet and each heat sink surface, plus the thickness of the cover glasssheet.

In some instances, the total thickness of the channel gap may be lessthan about (e.g., plus or minus 1%) 2500 μm (e.g., in the range fromabout 120 μm to about 2500 μm, about 150 μm to about 2500 μm, about 200μm to about 2500 μm, about 300 μm to about 2500 μm, about 400 μm toabout 2500 μm, about 500 μm to about 2500 μm, about 600 μm to about 2500μm, about 700 μm to about 2500 μm, about 800 μm to about 2500 μm, about900 μm to about 2500 μm, about 1000 μm to about 2500 μm, about 120 μm toabout 2250 μm, about 120 μm to about 2000 μm, about 120 μm to about 1800μm, about 120 μm to about 1600 μm, about 120 μm to about 1500 μm, about120 μm to about 1400 μm, about 120 μm to about 1300 μm, about 120 μm toabout 1200 μm, or about 120 μm to about 1000 μm). In some instances, thetotal thickness of the channel gap may be about 2500 μm or more (e.g.,in the range from about 2500 μm to about 10,000 μm, about 2500 μm toabout 9,000 μm, about 2500 μm to about 8,000 μm, about 2500 μm to about7,000 μm, about 2500 μm to about 6,000 μm, about 2500 μm to about 5,000μm, about 2500 μm to about 4,000 μm, about 2750 μm to about 10,000 μm,about 3000 μm to about 10,000 μm, about 3500 μm to about 10,000 μm,about 4000 μm to about 10,000 μm, about 4500 μm to about 10,000 μm, orabout 5000 μm to about 10,000 μm).

The apertures 331 a in the heat sink 331 may be positioned to beperpendicular to the heat sink surface or may be positioned at an angleof 20 degrees or less, such as about (e.g., plus or minus 1%) 15 degreesor less, about 10 degrees or less or about 5 degrees or less) fromperpendicular to the heat sink surface.

In some embodiments, the material behind the heat sink (cold bearing332) surfaces can be any suitable material having high heat transferrates, including metals (e.g., stainless steel, copper, aluminum),ceramics, carbon, etc. This material may be relatively thick compared tothe material behind the surfaces of the transition bearings 320, asshown in FIG. 22 , such that heat sink can easily accept relativelylarge amounts of thermal energy. In an exemplary embodiment, thematerial of the heat sinks 331 is stainless steel.

FIG. 23 is a cut-away perspective cross-section of an apparatus similarto that of FIG. 22 , albeit reversed from right to left, and comprisingadditionally a load/unload zone 340, next to cold zone 330 of system300, including a load/unload gas bearing 342 with a cover glass sheet400 c positioned thereon. Also, the apparatus of FIG. 23 uses tightchannel gaps (not indicated on the figure) in hot zone 310, transitionbearing 320, and cold zone 330.

The inset in FIG. 23 shows an alternative embodiment of a cold zone gasbearing 332 a, in which the gas bearing 322 a is actively cooled bycoolant channels 334, between gas bearing feed holes 333, where the feedholes feed the apertures in the surface of the bearing 322 a. Thecooling channels 334 are defined between heat sink segments 333 b, whichare assembled together to form the heat sink 331 and the surface thereoffacing the cover glass sheet 400 b.

The cooling channels 334 may be positioned very near the surface of theheat sink 331, in the solid material of the gas bearing 332, with aregion of solid bearing material between the heat sink/gas bearingsurface and the nearest-the-surface edge of the coolant channel 334,having the same width as the nearest-the-surface edge of the coolantchannel 334. Accordingly, in some embodiments there is no region ofreduced cross section in the solid material of the heat sink 331/gasbearing 332 a between a coolant channel 334 and the surface facing thecover glass 400 b. This differs from the typical convective gas coolingequipment, because the high gas flow rates mandate that significantspace be provided in the middle of the array of gas nozzles for the gasflows to escape. Where active cooling is used, heat sink 331/gas bearing332 a has a region of reduced cross section in the solid material of thegas nozzle design, relative to the solid material nearest the coverglass surface. The reduced cross section region is generally positionedbetween the active cooling fluid and cover glass sheet under treatment,in order to give a high-volume path for the large volume of heated gasreturning from the sheet.

FIG. 24 shows yet another alternative embodiment of a cold zone gasbearing 332, like that of the inset of FIG. 23 . In this embodiment,coolant channels 334 are formed between a gas bearing feed member 335,containing gas bearing feed holes 333, and a gas bearing face member 337a, which provides the cover glass sheet 400 b facing surface of the gasbearing 332. FIG. 25 shows yet another alternative cold zone gas bearing332 c having a similar structure to the embodiment of FIG. 24 , buthaving a porous member 339 between a bearing plate member 337 b andcover glass sheet 400 b, such that porous member 339 forms the surfacefacing the cover glass sheet 400 b.

It should be understood that in various embodiments, the cover glassstrengthening processes and systems described herein in relation toFIGS. 16-26 may be used or operated to form a cover glass or glassceramic article (such as cover glass sheet 500) having any combinationof features, characteristics, dimensions, physical properties, etc. ofany of the cover glass article embodiments discussed herein.

Cover glass sheets having undergone the thermal strengthening processesdescribed herein may be further processed by undergoing ion exchange tofurther enhance their strength. Ion-exchanging the surface of coverglasses heat strengthened as described herein may increase theabove-described compressive stresses by at least 20 MPa, such as atleast 50 MPa, such as at least 70 MPa, such as at least 80 MPa, such asat least 100 MPa, such as at least 150 MPa, such as at least 200 MPa,such as at least 300 MPa, such as at least 400 MPa, such as at least 500MPa, such as at least 600 MPa and/or no more than 1 GPa, in some suchcontemplated embodiments.

Systems and Processes for Thermal Conditioning and/or Heating CoverGlass Sheet

In addition to thermally strengthening thin cover glass sheets, theprocesses and systems described herein can be used for additionalthermal conditioning processes as well. While cooling is specificallydiscussed herein, the systems and processes can be used to transfer heatinto the cover glass sheet via a conductive method. Accordingly,additional embodiments of the processes of the current disclosure,including heating through a gas by conduction more than convection. Sucha process or method 700 is illustrated in the flow chart of FIG. 26 .

The method 700 includes two main steps. The first step, step 710,involves providing an article, such as a cover glass sheet, having atleast one surface. The second step, step 720, involves heating orcooling a portion of the surface of the article, up to and including theentire surface of the article. Step 720 is performed by conduction morethan by convection through a gas from or to a heat source or a heat sinksource as shown in sub-part 720 a, and is performed sufficiently tocomplete thermal conditioning of the article or the portion of thesurface of the article in sub-part 720 b, and the conduction of thecooling/heating of step 720 is performed at a high rate of heattransfer, at least 450 kW/m² of the area of the portion in sub-part 720b.

For example, an article can be thermally conditioned—i.e., either heatedor cooled—by cooling or heating a portion of the surface of the article,up to and including the entire surface of the article (the portionhaving an area), by conduction more than by convection, the conductionmediated through a gas to or from a heat sink or a heat source and notthrough solid-to-solid contact, sufficiently to complete a thermalconditioning of the article or of the portion of the surface of thearticle, and the conduction being performed, during at least some timeof the heating or cooling, at a rate of at least 450, 550, 650, 750,800, 900, 1000, 1100, 1200, 1500, 2000, 3000, 4000 or even 5000 or morekW per square meter.

In addition to tempering, the high rates of thermal power transferprovided by the systems and methods discussed herein allow for thermalprocessing or conditioning of all kinds, including heating and coolingduring tempering, edge strengthening of cover glass, firing or sinteringof ceramics, glasses, or other materials, and so forth. Additionally,since the heat is extracted or delivered primarily by conduction, tightcontrol is provided over the thermal history and the heat distributionin the treated article while preserving surface smoothness and quality.Accordingly, in yet another aspect of the present disclosure, tightcontrol is provided over the thermal history and the heat distributionin the treated article, since the heat is extracted or deliveredprimarily by conduction, yet surface smoothness and quality arepreserved. Accordingly, it will be possible to use the systems andmethods of the present disclosure to intentionally vary the stressprofile from the strengthening process, both in the thickness directionand in the directions in which the plane of the sheet lies, by varyinggaps, varying heat sink/source materials, varying heat sink/sourcetemperatures, varying the gas mixture—and all these may be varied byposition along the path of the sheet as it moves, or across the path ofthe sheet, or potentially in time also, not merely with position (formost of the variables).

Devices, Products and Structures Incorporating Strengthened Cover GlassSheets

The strengthened cover glass or glass-ceramic articles and sheetsdiscussed herein have a wide range of uses in a wide range of articles,devices, products, structures, etc. Discussion of cover glass orglass-ceramics herein is also referred to as consumer electronic glass.The cover glass or glass-ceramic of according to the present disclosuremay be used on any surface of electronic devices, mobile phones,portable media players, televisions, notebook computers, watches, userwearable devices (e.g., Fitbit), cameras lenses, camera displays,household appliances, tablet computer displays, and any other electronicdevices which may require a surface according to the properties anddimensions described herein.

Referring to FIG. 27 , a structure 1010, such as a building, house,vehicle, etc., includes a glass or glass-ceramic article 1012 in theform of a window, portion of walls (e.g., surfaces), dividers, etc. Incontemplated embodiments, the glass or ceramic article 1012 may bestrengthened such that the glass or ceramic article 1012 has a negativetensile stress on or near surfaces thereof, balanced by a positivetensile stress internal thereto, as disclosed herein. Further, the glassor glass-ceramic article 1012 may have a composition that resistschemicals and/or corrosion as may be present in outdoor environments byhaving a relatively high silicon dioxide content, such as at least 70%silicon dioxide by weight, such as at least 75% by weight.

According to an exemplary embodiment, the glass or glass-ceramic article1012 has major surfaces orthogonal to a thickness thereof (see generallysheet 500 as shown in FIG. 4 ), where the major surfaces have a largearea (e.g., at least 5 cm², at least 9 cm², at least 15 cm², at least 50cm², at least 250 cm²) relative to glass or glass-ceramic articles usedin other applications (e.g., lenses, battery components, etc.). Incontemplated embodiments, total light transmission through the glass orglass-ceramic articles 1012 is at least about 50% (e.g., at least 65%,at least 75%) from wavelengths of about 300 nm to about 800 nm, when theglass or glass ceramic article 1012 has thicknesses as disclosed herein,such as a thickness of less than 5 cm, less than 3 cm, less than 2 cm,less than 1.75 cm, less than 1.5 cm, less than 1 cm, less than 5 mm,less than 3 mm, less than 2 mm, less than 1.75 mm, less than 1.5 mm,less than 1 mm, less than 0.8 mm, less than 0.6 mm, less than 0.5 mm,less than 0.4 mm, less than 0.2 mm, and/or at least 10 micrometers, suchas at least 50 micrometers.

Thin thicknesses of the glass or glass-ceramic article 1012 may not harmthe function of the glass or glass-ceramic article 1012 inarchitectural, automotive, or other applications relative toconventional articles because the high level of strength of the glass orglass-ceramic article 1012 provided by the inventive processes disclosedherein. Thin glass or glass-ceramic articles 1012 may be particularlyuseful in such architectural, automotive, consumer electronics, or otherapplications because the glass or glass ceramic article 1012 may belighter than conventional such articles, reducing the weight of thecorresponding overall structure. For automobiles, a result may begreater fuel efficiency. For buildings, a result may be sturdier or lessresource-intensive structures. For consumer electronics, a lighterdevice with greater impact resistance and/or resilience to recurringdrops or impacts. In other contemplated embodiments, glass orglass-ceramic articles disclosed herein may have areas of lessermagnitude, greater thicknesses, transmit less light, and/or may be usedin different applications, such as those disclosed with regard to FIGS.27-30 , for example.

Referring to FIG. 28 , a surface 1110 includes a glass or glass ceramicarticle 1112, manufactured as disclosed herein and/or with anycombination of stress profiles, structures and/or physical propertiesdiscussed herein, that functions as a countertop and/or as a portion ofa display. In some embodiments, total transmission through the coverglass or glass ceramic articles 1012 is at least about 30% (e.g., atleast 50%) from infrared wavelengths of about 800 nm to about 1500 nm,facilitating use of the surface 1110 as a cooktop. In some embodiments,the cover glass or glass-ceramic article 1112 has a coefficient ofthermal expansion (CTE) from about 10×10⁻⁷° C.⁻¹ to about 140×10⁻⁷°C.⁻¹, about 20×10⁻⁷° C.⁻¹ to about 120×10⁻⁷° C.⁻¹, about 30×10⁻⁷° C.⁻¹to about 100×10⁻⁷° C.⁻¹, about 40×10⁻⁷° C.⁻¹ to about 100×10⁻⁷° C.⁻¹,about 50×10⁻⁷° C.⁻¹ to about 100×10⁻⁷° C.⁻¹, or about 60×10⁻⁷° C.⁻¹ toabout 120×10⁻⁷° C. In various embodiments, the processes are ideallysuited for glass compositions having moderate to high CTEs. Examplecover glasses that work well with the processes described herein includealkali aluminosilicates, such as Corning's® Gorilla® Glasses,boroaluminosilicates, and soda-lime glasses. In some embodiments, thecover glasses used have CTEs greater than 40, greater than 50, greaterthan 60, greater than 70, greater than 80, or greater than 90×10⁻⁷/° C.Some such CTEs may be particularly low for thermal tempering asdisclosed herein, where the degree of negative tensile stress is no morethan 50 MPa and/or at least 10 MPa.

Referring to FIG. 29 , a device 1210 (e.g., handheld computer, tablet,portable computer, cellular phone, television, watch, display board,etc.) includes one or more cover glass or glass-ceramic articles 1212,1214, 1216, manufactured as disclosed herein and/or with any combinationof stress profiles, structures and/or physical properties as disclosedherein, and further includes electronic components 1218 (e.g., adisplay, and electrical display, a controller, a memory, a microchip,etc.) and a housing 1220. In embodiments, electrical components 1218and/or the electrical display may include a liquid crystal displayand/or at least one light emitting diode (LED). In embodiments, theelectronic display may be a touch sensitive display. In furtherembodiments, the glass-based layer forming or covering the electronicdisplay may include a surface feature on the first or second majorsurface for haptic feedback for a user. For example, raised projections,ridges, contours, or bumps are non-limiting example surface features forhaptic feedback. In embodiments, electrical components 1218 are providedat least partially within housing 1220. In embodiments, electricalcomponents 1218 are provided completely within housing 1220. Incontemplated embodiments, the housing 1220 may be or include a coverglass or glass-ceramic article as disclosed herein. In contemplatedembodiments, a substrate 1222 for the electronic components 1218 may bea cover glass or glass-ceramic article as disclosed herein.

In some embodiments, the cover glass or glass ceramic articles 1212,1214 may function as frontplane and backplane substrates, and the coverglass or glass ceramic article 1216 may function as a cover glass in thedevice 1210. According to an exemplary embodiment, the cover glass orglass-ceramic article 1216 of the device 1210 is analkali-aluminosilicate glass. Such composition may allow the cover glassor glass-ceramic article 1216 to be strengthened by thermal tempering,as disclosed herein, and may be additionally strengthened byion-exchange, providing a particularly high degree of negative tensilestress (e.g., at least 200 MPa, at least 250 MPa) at or near surfacesthereof. In other embodiments, the cover glass or glass-ceramic article1216 may include sodium carbonate, calcium oxide, calcium magnesiumcarbonate, silicon dioxide (e.g., at least 70% by weight), aluminumoxide, and/or other constituents; and may be strengthened by theinventive processes disclosed herein. The cover glass or glass ceramicarticle 1216 may be particularly thin or otherwise structured, such ashaving any of the dimensions, properties, and/or compositions asdisclosed herein.

In embodiments, housing 1220 may include a front surface, a backsurface, and at least one side surface 1220. Housing 1220 may includeone or more glass-based layers including cover glass or glass-ceramicarticles manufactured as disclosed herein and/or with any combination ofstress profiles, structures and/or physical properties discussed herein.In embodiments, the glass-based layer may be a cover glass orglass-ceramic article as disclosed herein. The glass based layer (e.g.,1212, 1214, 1216) may form any surface of a consumer electronic product.In one or more embodiments, the glass-based layer extends across thehousing front surface from at least one side surface (e.g., 1220) to anopposite side surface. In embodiments, the glass-based layer is providedat or adjacent the front surface of housing 1220. In furtherembodiments, the glass-based layer may include a surface feature on thefirst or second major surface for haptic feedback for a user. Forexample, raised projections, ridges, contours, or bumps are non-limitingexample surface features for haptic feedback. In embodiments, glassbased layer (e.g., 1212, 1214, 1216) may be shaped in 1-dimension,2-dimensions, 2.5-dimensions (e.g., curvature at the edge of a displayglass), or 3-dimensions.

In other embodiments, the glass-based layer (e.g., 1212, 1214, 1216) mayhave at least one beveled or curved edge, including an embodiment wherethe entire outside perimeter of the glass-based layer is beveled orcurved. In embodiments, the average thickness of the glass-based layermay not exceed 1.5 mm, may not exceed 1.0 mm, may not exceed 0.7 mm, maynot exceed 0.5 mm, or may have an average thickness within a range fromabout 0.5 mm to about 1.0 mm, or about 0.1 mm to about 1.5 mm, or anaverage thickness from about 0.5 mm to about 0.7 mm. In yet otherembodiments, the one or more of the major surfaces of the glass-basedlayer may include an anti-scratch layer, an antireflection layer, and anantiglare layer. The one or more major surfaces of the glass-based layermay also include any combination or all of these layers.

The cover glass or glass-ceramic article may include a glass materialthat is substantially optically clear, transparent and free from lightscattering. In such embodiments, the cover glass material may exhibit anaverage light transmission over a wavelength range from about 400 nm toabout 780 nm of about 85% or greater, about 86% or greater, about 87% orgreater, about 88% or greater, about 89% or greater, about 90% orgreater, about 91% or greater or about 92% or greater. In one or morealternative embodiments, the glass material may be opaque or exhibit anaverage light transmission over a wavelength range from about 400 nm toabout 780 nm of less than about 10%, less than about 9%, less than about8%, less than about 7%, less than about 6%, less than about 5%, lessthan about 4%, less than about 3%, less than about 2%, less than about1%, or less than about 0%. In some embodiments, these light reflectanceand transmittance values may be a total reflectance or totaltransmittance (taking into account reflectance or transmittance on bothmajor surfaces of the glass material). The glass material may optionallyexhibit a color, such as white, black, red, blue, green, yellow, orange,etc.

Referring now to FIG. 30 , a cover glass or glass-ceramic article 1310,manufactured according to processes disclosed herein and/or with anycombination of stress profiles, structures and/or physical properties asdisclosed herein, has curvature and/or a variable cross-sectionaldimension D. Such articles may have thicknesses disclosed herein as anaverage of dimension D or as a maximum value of dimension D. While thecover glass or glass-ceramic article 1310 is shown as a curved sheet,other shapes, such as more complex shapes, may be strengthened byprocesses disclosed herein. In contemplated embodiments, the cover glassor glass ceramic article 1310 may be used as a front pane, back pane, oron any surface of a consumer electronic product.

In various embodiments, cover glass material manufactured according toprocesses disclosed herein, and/or with any combination of stressprofiles, structures and/or physical properties as disclosed herein, isuseful to form at least one sheet of a cover glass-interlayer-coverglass laminate, such as used in automotive glass sidelights. Strongerand thinner laminates can be produced, resulting in weight and costsavings and fuel efficiency increases. Desirably, a thermallystrengthened thin sheet may be cold bent (see generally FIG. 30 ) andlaminated to a formed thicker glass, providing an easy and reliablemanufacturing process not requiring any hot forming/shaping of the thinsheet.

Glass and Glass Ceramic Materials for Thermally Strengthened Cover GlassSheets

The systems and methods discussed may be used to thermally condition,strengthen and/or temper a wide variety of cover glass and/or ceramicmaterials.

The processes and systems described herein may generally be used withalmost any glass composition, and some embodiments can be used withglass laminates, glass ceramics, and/or ceramics. In variousembodiments, the processes can be used with glass compositions havinghigh CTEs. In embodiments, cover glasses strengthened via the processesand systems discussed herein include alkali aluminosilicates, such asCorning's® Gorilla® Glasses, SLG, soda- or alkali-free glasses and thelike. In some embodiments, cover glasses strengthened via the processesand systems discussed herein have CTEs of greater than 40×10⁻⁷/° C.,greater than 50×10⁻⁷/° C., greater than 60×10⁻⁷/° C., greater than70×10⁻⁷/° C., greater than 80×10⁻⁷/° C., or greater than 90×10⁻⁷/° C.

In some applications and embodiments, cover glasses strengthened via theprocesses and systems discussed herein (such as cover glass sheet 500)may have a composition configured for chemical durability. In some suchembodiments, the composition comprises at least 70% silicon dioxide byweight, and/or at least 10% sodium oxide by weight, and/or at least 7%calcium oxide by weight. Conventional articles of such compositions maybe difficult to chemically temper to a deep depth, and/or may bedifficult, if not impossible, to thermally temper by conventionalprocesses to a sufficient magnitude of negative surface tensile stressfor thin thicknesses, such as due to fragility and forces ofconventional processes. However, in contemplated embodiments, inventiveprocesses disclosed herein allow a strengthened cover glass orglass-ceramic article or sheet, such as cover glass sheet 500, with sucha composition, where negative tensile stress extends into the respectivestrengthened cover glass or glass-ceramic sheet to a distance of atleast 10% of the thickness of the strengthened cover glass orglass-ceramic sheet from at least one of the first and second surfaces(e.g., surfaces 510, 520 of cover glass sheet 500), such as at least 12%of the thickness, 15% of the thickness, 16% of the thickness, 17% of thethickness, 18% of the thickness, 19% of the thickness, 20% of thethickness, or 21% of the thickness.

In some embodiments, the cover glass or glass-ceramic sheets andarticles strengthened as discussed herein have one or more coatings thatare placed on the cover glass prior to the thermal strengthening of thecover glass sheet. The processes discussed herein can be used to producestrengthened cover glass sheets having one or more coatings, and, insome such embodiments, the coating is placed on the cover glass prior tothermal strengthening and is unaffected by the thermal strengtheningprocess. Specific coatings that are advantageously preserved on coverglass sheets of the present disclosure include low E coatings,reflective coatings, antireflective coatings, anti-fingerprint coatings,cut-off filters, pyrolytic coatings, etc.

According to an exemplary embodiment, cover glass or glass-ceramicsheets or articles discussed herein, for example articles 1212, 1214 ofthe device 1210 shown in FIG. 29 , are boro-aluminosilicate glasses. Insome embodiments cover glass or glass ceramic sheets or articlesdiscussed herein, for example articles 1212, 1214 of the device 1210shown in FIG. 29 , are generally non-alkali glasses, yet still havestress profiles and structures as disclosed herein. Such composition mayreduce the degree of relaxation of the glass, facilitating coupling oftransistors thereto. In some embodiments, the cover glasssheets/articles discussed herein are flexible glass sheets. In otherembodiments, the cover glass sheets/articles discussed herein comprise alaminate of two or more cover glass sheets.

In some contemplated embodiments, cover glasses strengthened via theprocesses and systems discussed herein (such as cover glass sheet 500)may include an amorphous substrate, a crystalline substrate or acombination thereof, such as a glass-ceramic substrate. Cover glassesstrengthened via the processes and systems discussed herein (such ascover glass sheet 500) may include an alkali aluminosilicate glass,alkali containing borosilicate glass, alkali aluminophosphosilicateglass or alkali aluminoborosilicate glass. In one or more embodiments,cover glasses strengthened via the processes and systems discussedherein (such as cover glass sheet 500), in portions thereof notion-exchanged, may include a cover glass having a composition, in molepercent (mol %), including: SiO₂ in the range from about (e.g., plus orminus 1%) 40 to about 80 mol %, Al₂O₃ in the range from about 10 toabout 30 mol %, B₂O₃ in the range from about 0 to about 10 mol %, R₂O inthe range from about 0 to about 20 mol %, and/or RO in the range fromabout 0 to about 15 mol %. In some contemplated embodiments, thecomposition may include either one or both of ZrO₂ in the range fromabout 0 to about 5 mol % and P₂O₅ in the range from about 0 to about 15mol %. In some contemplated embodiments, TiO₂ can be present from about0 to about 2 mol %.

In some contemplated embodiments, compositions used for the strengthenedcover glass or glass-ceramic sheet or article discussed herein may bebatched with 0-2 mol % of at least one fining agent selected from agroup that includes Na₂SO₄, NaCl, NaF, NaBr, K₂SO₄, KCl, KF, KBr, andSnO₂. The cover glass composition according to one or more embodimentsmay further include SnO₂ in the range from about 0 to about 2 mol %,from about 0 to about 1 mol %, from about 0.1 to about 2 mol %, fromabout 0.1 to about 1 mol %, or from about 1 to about 2 mol %. Coverglass compositions disclosed herein for the strengthened cover glass orglass-ceramic sheet 500 may be substantially free of As₂O₃ and/or Sb₂O₃,in some embodiments.

In contemplated embodiments, the strengthened cover glass orglass-ceramic sheet or article discussed herein may include alkalialuminosilicate cover glass compositions or alkali aluminoborosilicateglass compositions that are further strengthened via an ion exchangeprocess. One example cover glass composition comprises SiO₂, B₂O₃ andNa₂O, where (SiO₂+B₂O₃)≥66 mol. %, and/or Na₂O≥9 mol. %. In anembodiment, the cover glass composition includes at least 6 wt. %aluminum oxide. In a further embodiment, the strengthened cover glass orglass-ceramic sheet or article discussed herein may include a glasscomposition with one or more alkaline earth oxides, such that a contentof alkaline earth oxides is at least 5 wt. %. Suitable cover glasscompositions, in some embodiments, further comprise at least one of K₂O,MgO and CaO. In a particular embodiment, the cover glass compositionsused in the strengthened cover glass or glass-ceramic sheet or articlediscussed herein can comprise 61-75 mol. % SiO₂; 7-15 mol. % Al₂O₃; 0-12mol. % B₂O₃; 9-21 mol. % Na₂O; 0-4 mol. % K₂O; 0-7 mol. % MgO; and/or0-3 mol. % CaO.

A further example cover glass composition suitable for the strengthenedcover glass or glass-ceramic sheet or article discussed hereincomprises: 60-70 mol. % SiO₂; 6-14 mol. % Al₂O₃; 0-15 mol. % B₂O₃; 0-15mol. % Li₂O; 0-20 mol. % Na₂O; 0-10 mol. % K₂O; 0-8 mol. % MgO; 0-10mol. % CaO; 0-5 mol. % ZrO₂; 0-1 mol. % SnO₂; 0-1 mol. % CeO₂; less than50 ppm As₂O₃; and less than 50 ppm Sb₂O₃; where 12 mol. %(Li₂O+Na₂O+K₂O)≤20 mol. % and/or 0 mol. % (MgO+CaO)≤10 mol. %. A stillfurther example glass composition suitable for the strengthened coverglass or glass-ceramic sheet or article discussed herein comprises:63.5-66.5 mol. % SiO₂; 8-12 mol. % Al₂O₃; 0-3 mol. % B₂O₃; 0-5 mol. %Li₂O; 8-18 mol. % Na₂O; 0-5 mol. % K₂O; 1-7 mol. % MgO; 0-2.5 mol. %CaO; 0-3 mol. % ZrO₂; 0.05-0.25 mol. % Sn₂O₂; 0.05-0.5 mol. % CeO₂; lessthan 50 ppm As₂O₃; and less than 50 ppm Sb₂O₃; where 14 mol.%≤(Li₂O+Na₂O+K₂O)≤18 mol. % and/or 2 mol. % (MgO+CaO)≤7 mol. %.

In particular contemplated embodiments, an alkali aluminosilicate glasscomposition suitable for the strengthened cover glass or glass-ceramicsheet or article discussed herein comprises alumina, at least one alkalimetal and, in some embodiments, greater than 50 mol. % SiO₂, in otherembodiments at least 58 mol. % SiO₂, and in still other embodiments atleast 60 mol. % SiO₂, wherein the ratio (Al₂O₃+B₂O₃)/Σmodifiers (i.e.,sum of modifiers) is greater than 1, where in the ratio the componentsare expressed in mol. % and the modifiers are alkali metal oxides. Thiscover glass composition, in particular embodiments, comprises: 58-72mol. % SiO₂; 9-17 mol. % Al₂O₃; 2-12 mol. % B₂O₃; 8-16 mol. % Na₂O;and/or 0-4 mol. % K₂O, wherein the ratio (Al₂O₃+B₂O₃)/Σmodifiers (i.e.,sum of modifiers) is greater than 1. In still another embodiment, thestrengthened cover glass or glass-ceramic sheet 500 may include analkali aluminosilicate glass composition comprising: 64-68 mol. % SiO₂;12-16 mol. % Na₂O; 8-12 mol. % Al₂O₃; 0-3 mol. % B₂O₃; 2-5 mol. % K₂O;4-6 mol. % MgO; and 0-5 mol. % CaO, wherein: 66 mol. %≤SiO₂+B₂O₃+CaO≤69mol. %; Na₂O+K₂O+B₂O₃+MgO+CaO+SrO>10 mol. %; 5 mol. % MgO+CaO+SrO≤8 mol.%; (Na₂O+B₂O₃)—Al₂O₃≤2 mol. %; 2 mol. %≤Na₂O—Al₂O₃≤6 mol. %; and 4 mol.%≤(Na₂O+K₂O)—Al₂O₃10 mol. %. In an alternative embodiment, thestrengthened cover glass or glass-ceramic sheet or articles discussedherein may comprise an alkali aluminosilicate glass compositioncomprising: 2 mol. % or more of Al₂O₃ and/or ZrO₂, or 4 mol. % or moreof Al₂O₃ and/or ZrO₂.

In contemplated embodiments, examples of suitable glass ceramics for thestrengthened cover glass or glass-ceramic sheet or articles discussedherein may include Li₂O—Al₂O₃—SiO₂ system (i.e. LAS-System) glassceramics, MgO—Al₂O₃—SiO₂ system (i.e. MAS-System) glass ceramics, and/orglass ceramics that include a predominant crystal phase includingβ-quartz solid solution, β-spodumene ss, cordierite, and lithiumdisilicate. The strengthened cover glass or glass-ceramic sheet orarticle discussed herein may be characterized by the manner in which itis formed. For instance, the strengthened cover glass or glass-ceramicsheet or article discussed herein may be characterized as float-formable(i.e., formed by a float process), down-drawable and, in particular,fusion-formable or slot-drawable (i.e., formed by a down draw processsuch as a fusion draw process or a slot draw process).

A float-formable strengthened cover glass or glass-ceramic sheet orarticle may be characterized by smooth surfaces and consistentthickness, and is made by floating molten cover glass on a bed of moltenmetal, typically tin. In an example process, molten cover glass orglass-ceramic that is fed onto the surface of the molten tin bed forms afloating glass or glass-ceramic ribbon. As the cover glass ribbon flowsalong the tin bath, the temperature is gradually decreased until thecover glass or glass-ceramic ribbon solidifies into a solid cover glassor glass-ceramic article that can be lifted from the tin onto rollers.Once off the bath, the cover glass or glass-ceramic article can becooled further and annealed to reduce internal stress. Where the coverglass or glass-ceramic article is a glass ceramic, the cover glassarticle formed from the float process may be subjected to a cerammingprocess by which one or more crystalline phases are generated.

Down-draw processes produce cover glass or glass-ceramic articles havinga consistent thickness that possess relatively pristine surfaces.Because the average flexural strength of the cover glass orglass-ceramic article is controlled by the amount and size of surfaceflaws, a pristine surface that has had minimal contact has a higherinitial strength. When this high strength cover glass or glass-ceramicarticle is then further strengthened (e.g., chemically), the resultantstrength can be higher than that of a cover glass or glass-ceramicarticle with a surface that has been lapped and polished. Down-drawncover glass or glass-ceramic articles may be drawn to a thickness ofless than about 2 mm. In addition, down-drawn cover glass orglass-ceramic articles have a very flat, smooth surface that can be usedin its final application without costly grinding and polishing. Wherethe cover glass or glass-ceramic article is a glass ceramic, the coverglass or glass-ceramic article formed from the down-draw process may besubjected to a ceramming process by which one or more crystalline phasesare generated.

The fusion draw process, for example, uses a drawing tank that has achannel for accepting molten glass raw material. The channel has weirsthat are open at the top along the length of the channel on both sidesof the channel. When the channel fills with molten material, the moltenglass overflows the weirs. Due to gravity, the molten glass flows downthe outside surfaces of the drawing tank as two flowing glass films.These outside surfaces of the drawing tank extend down and inwardly sothat they join at an edge below the drawing tank. The two flowing glassfilms join at this edge to fuse and form a single flowing cover glassarticle. The fusion draw method offers the advantage that, because thetwo cover glass films flowing over the channel fuse together, neither ofthe outside surfaces of the resulting cover glass article comes incontact with any part of the apparatus. Thus, the surface properties ofthe fusion drawn cover glass article are not affected by such contact.Where the cover glass or glass-ceramic article is a glass ceramic, thecover glass or glass-ceramic article formed from the fusion process maybe subjected to a ceramming process by which one or more crystallinephases are generated.

The slot draw process is distinct from the fusion draw method. In slotdraw processes, the molten raw material glass is provided to a drawingtank. The bottom of the drawing tank has an open slot with a nozzle thatextends the length of the slot. The molten glass flows through theslot/nozzle and is drawn downward as a continuous cover glass articleand into an annealing region. Where the cover glass or glass-ceramicarticle is a glass ceramic, the cover glass article formed from the slotdraw process may be subjected to a ceramming process by which one ormore crystalline phases are generated.

In some embodiments, the cover glass article may be formed using a thinrolling process, as described in U.S. Pat. Nos. 8,713,972, 9,003,835,U.S. Patent Publication No. 2015/0027169, and U.S. Patent PublicationNo. 20050099618, the contents of which are incorporated herein byreference in their entirety. More specifically the cover glass orglass-ceramic article may be formed by supplying a vertical stream ofmolten glass, forming the supplied stream of molten glass orglass-ceramic with a pair of forming rolls, maintained at a surfacetemperature of about 500° C. or higher or about 600° C. or higher, toform a formed cover glass ribbon having a formed thickness, sizing theformed ribbon of glass with a pair of sizing rolls, maintained at asurface temperature of about 400° C. or lower to produce a sized glassribbon having a desired thickness less than the formed thickness and adesired thickness consistency. The apparatus used to form the coverglass ribbon may include a glass feed device for supplying a suppliedstream of molten glass; a pair of forming rolls maintained at a surfacetemperature of about 500° C. or higher, the forming rolls being spacedclosely adjacent each other, defining a glass forming gap between theforming rolls with the glass forming gap located vertically below theglass feed device for receiving the supplied stream of molten glass andthinning the supplied stream of molten glass between the forming rollsto form a formed glass ribbon having a formed thickness; and a pair ofsizing rolls maintained at a surface temperature of about 400° C. orlower, the sizing rolls being spaced closely adjacent each other,defining a glass sizing gap between the sizing rolls with the coverglass sizing gap located vertically below the forming rolls forreceiving the formed cover glass ribbon and thinning the formed coverglass ribbon to produce a sized cover glass ribbon having a desiredthickness and a desired thickness consistency.

In some instances, the thin rolling process may be utilized where theviscosity of the glass does not permit use of fusion or slot drawmethods. For example, thin rolling can be utilized to form the coverglass or glass-ceramic articles when the glass exhibits a liquidusviscosity less than 100 kP. The cover glass or glass-ceramic article maybe acid polished or otherwise treated to remove or reduce the effect ofsurface flaws.

In contemplated embodiments, the cover glass or glass-ceramic sheet orarticle discussed herein has a composition that differs by side surface.On one side of the cover glass or glass-ceramic sheet 500, an exemplarycomposition is: 69-75 wt. % SiO₂, 0-1.5 wt. % Al₂O₃, 8-12 wt. % CaO,0-0.1 wt. % Cl, 0-500 ppm Fe, 0-500 ppm K, 0.0-4.5 wt. % MgO, 12-15 wt.% Na₂O, 0-0.5 wt. % SO₃, 0-0.5 wt. % SnO₂, 0-0.1 wt. % SrO, 0-0.1 wt. %TiO₂, 0-0.1 wt. % ZnO, and/or 0-0.1 wt. % ZrO₂. On the other side of thecover glass or glass-ceramic sheet or article discussed herein anexemplary composition is: 73.16 wt. % SiO₂, 0.076 wt. % Al₂O₃, 9.91 wt.% CaO, 0.014 wt. % Cl, 0.1 wt. % Fe₂O₃, 0.029 wt. % K₂O, 2.792 wt. %MgO, 13.054 wt. % Na₂O, 0.174 wt. % SO₃, 0.001 SnO₂, 0.01 wt. % SrO,0.01 wt. % TiO₂, 0.002 wt. % ZnO, and/or 0.005 wt. % ZrO₂.

In other contemplated embodiments, composition of the cover glass orglass-ceramic sheet or article discussed herein includes SiO₂ 55-85 wt.%, Al₂O₃ 0-30 wt. %, B₂O₃ 0-20 wt. %, Na₂O 0-25 wt. %, CaO 0-20 wt. %,K₂O 0-20 wt. %, MgO 0-15 wt. %, BaO 5-20 wt. %, Fe₂O₃ 0.002-0.06 wt. %,and/or Cr₂O₃ 0.0001-0.06 wt. %. In other contemplated embodiments,composition of the cover glass or glass-ceramic sheet or articlediscussed herein includes SiO₂ 60-72 mol. %, Al₂O₃ 3.4-8 mol. %, Na₂O13-16 mol. %, K₂O 0-1 mol. %, MgO 3.3-6 mol. %, TiO₂ 0-0.2 mol. %, Fe₂O₃0.01-0.15 mol. %, CaO 6.5-9 mol. %, and/or SO₃ 0.02-0.4 mol. %.

EXAMPLES

Apparatus setup—As detailed above, the apparatus comprises three zones—ahot zone, a transition zone, and a cool or quench zone. The gaps betweenthe top and bottom thermal bearings (heat sinks) in the hot zone and thequench zone are set to the desired spacings. Gas flow rates in the hotzone, transition zone, and quench zone are set to ensure centering ofthe glass material, sheet or part on the air-bearing. The hot zone ispre-heated to the desired T₀, the temperature from which the glassarticle will be subsequently quenched. To ensure uniform heating, coverglass articles are pre-heated in a separate pre-heating apparatus, suchas a batch or continuous furnace. Generally, cover glass sheets arepre-heated for greater than 5 minutes prior to loading in the hot zone.For soda-lime glasses, pre-heating is done around 450° C. After thepre-heat phase, the cover glass article is loaded into the hot zone andallowed to equilibrate, where equilibration is where the glass isuniformly at T₀. T₀ can be determined by the level ofstrengthening/tempering desired, but is generally kept in the rangebetween the softening point and the glass transition temperature. Thetime to equilibration is dependent at least on the thickness of thecover glass. For example, for cover glass sheets of approximately 1.1 mmor less, equilibration occurs in approximately 10 seconds. For 3 mmcover glass sheets, equilibration occurs in approximately 10 seconds to30 seconds. For thicker sheets, up to approximately 6 mm, theequilibration time may be on the order of 60 seconds. Once the coverglass has equilibrated to T₀, it is rapidly transferred through thetransition zone on air bearings and into the cool or quench zone. Thecover glass article rapidly quenches in the quench zone to a temperaturebelow the glass transition temperature, Tg. The cover glass sheet can bemaintained in the quench zone for any period of time from 1 second, 10seconds, or to several minutes or more, depending on the degree ofquench desired and/or the desired temperature of the cover glass atremoval. Upon removal the cover glass is optionally allowed to coolbefore handling.

The following examples are summarized in Table VI.

Example 1—A soda-lime silicate glass plate (e.g., glass comprising atleast 70% silicon dioxide by weight, and/or at least 10% sodium oxide byweight, and/or at least 7% calcium oxide by weight) of 5.7 mm thicknessis pre-heated for 10 minutes at 450° C. before transferring to the hotzone where it is held at a T₀ of 690° C. for 60 seconds. Afterequilibrating to T₀, it is rapidly transferred to the quench zone filledwith helium, which has a gap of 91 μm (wherein the gap is the distancebetween the surface of the glass sheet and the nearest heat sink), whereit is held for 10 seconds. The resulting article has a surfacecompression of −312 MPa, a central tension of 127 MPa, and a flatness of83 μm.

Example 2—A soda-lime silicate glass plate of 5.7 mm thickness ispre-heated for 10 minutes at 450° C. before transferring to the hot zonewhere it is held at a T₀ of 690° C. for 60 seconds. After equilibratingit is rapidly transferred to the quench zone, which has a gap of 91 μm,where it is held for 10 seconds. The resulting article has a surfacecompression of −317 MPa, a central tension of 133 MPa, and a flatness ofabout 89.7 micrometers.

Example 3—A soda-lime silicate glass plate of 1.1 mm thickness ispre-heated for 10 minutes at 450° C. before transferring to the hot zonewhere it is held at a T₀ of 700° C. for 10 seconds. After equilibratingit is rapidly transferred to the quench zone filled with helium, whichhas a gap of 56 μm, where it is held for 10 seconds. The resultingarticle has a surface fictive temperature measured to be 661° C., asurface compression of −176 MPa, a central tension of 89 MPa, a flatnessof 190 μm, and a Vicker's cracking threshold of 10-20 N.

Example 4—A soda-lime silicate glass plate of 0.55 mm thickness ispre-heated for 10 minutes at 450° C. before transferring to the hot zonewhere it is held at a T₀ of 720° C. for 10 seconds. After equilibratingit is rapidly transferred to the quench zone, which has a gap of 25 μm,where it is held for 10 seconds, resulting in an effective heat transferrate of 0.184 cal/(cm²−s−° C.). The resulting article has a surfacecompression of −176 MPa and a central tension of 63 MPa. Also, theresulting strengthened articles had a flatness of about 168 (for theinitial 710° C. temperature sample) and 125 micrometers (for the initial720° C. temperature sample).

Example 5—A CORNING® GORILLA® Glass plate of 1.5 mm thickness ispre-heated for 10 minutes at 550° C. before transferring to the hot zonewhere it is held at a T₀ of 790° C. for 30 seconds. After equilibratingit is rapidly transferred to the quench zone, which has a gap of 226 μm,where it is held for 10 seconds. The glass article has an improvement inflatness measured to be 113 μm pre-processing and 58 μm post-processing.

Example 6—A soda-lime silicate glass plate of 0.7 mm thickness ispre-heated for 10 minutes at 450° C. before transferring to the hot zonewhere it is held at a T₀ of 730° C. for 10 seconds. After equilibratingit is rapidly transferred to the quench zone filled with helium, whichhas a gap of 31 μm, where it is held for 10 seconds, resulting in aneffective heat transfer rate of 0.149 cal/(cm²−s−° C.). The resultingarticle has a surface compression of −206 MPa, a central tension of 100MPa, and a flatness of 82 μm. Upon fracture, the glass sheet is observedto “dice” (using standard terminology for 2 mm thickness or greatersheet dicing—i.e., a 5×5 cm square of glass sheet breaks into 40 or morepieces) suggesting that the sheet is fully tempered.

Example 7—A Borofloat-33 glass plate of 3.3 mm thickness is pre-heatedfor 10 minutes at 550° C. before transferring to the hot zone where itis held at a T₀ of 800° C. for 30 seconds. After equilibrating it israpidly transferred to the quench zone, which has a gap of 119 μm, whereit is held for 10 seconds. The resulting article has a flatness of 120μm. Upon fracture of the part it is observed to “dice” (using standardterminology for 2 mm or greater thickness sheet dicing—i.e., a 5×5 cmsquare of glass sheet breaks into 40 or more pieces) showing that thesheet is fully tempered.

Example 8—A soda-lime silicate glass plate of 3.2 mm thickness ispre-heated for 10 minutes at 450° C. before transferring to the hot zonewhere it is held at a T₀ of 690° C. for 30 seconds. After equilibratingit is rapidly transferred to the quench zone, which has a gap of 84 μm,where it is held for 10 seconds. The resulting article has a surfacecompression of −218 MPa, a central tension of 105 MPa, and a flatness of84 μm.

Example 9—A soda-lime silicate glass plate of 0.3 mm thickness ispre-heated for 10 minutes at 450° C. before transferring to the hot zonewhere it is held at a T₀ of 630° C. for 10 seconds. After equilibratingit is rapidly transferred to the quench zone, which has a gap of 159 μm,where it is held for 10 seconds. The resulting article has membranestresses which are observable by gray field polarimetry, suggesting theglass has incorporated the thermal stress.

Example 10—A CORNING® GORILLA® Glass plate of 0.1 mm thickness ispre-heated for 10 minutes at 550° C. before transferring to the hot zonewhere it is held at a T₀ of 820° C. for 10 seconds. After equilibratingit is rapidly transferred to the quench zone, which has a gap of 141 μm,where it is held for 10 seconds, resulting in an effective heat transferrate of 0.033 cal/(cm²−s−° C.). Upon fracture, the resulting articledisplays behavior consistent with a residually stressed glass.

Example 11—A soda-lime silicate glass plate of 1.1 mm thickness ispre-heated for 10 minutes at 450° C. before transferring to the hot zonewhere it is held at a T₀ of 700° C. for 10 seconds. After equilibratingit is rapidly transferred to the quench zone, which has a gap of 65 μm,where it is held for 10 seconds, resulting in an effective heat transferrate of 0.07 cal/(cm²−s−° C.). The resulting article has a surfacefictive temperature measured to be 657° C., a surface compression of−201 MPa, a central tension of 98 MPa, a flatness of 158 μm, and aVicker's cracking threshold of 10-20 N.

Example 12—A CORNING® GORILLA® Glass plate of 1.1 mm thickness ispre-heated for 10 minutes at 550° C. before transferring to the hot zonewhere it is held at a T₀ of 810° C. for 10 seconds. After equilibratingit is rapidly transferred to the quench zone which has a gap of 86 μm,where it is held for 10 seconds, resulting in an effective heat transferrate of 0.058 cal/(cm²−s−° C.). The resulting article has a surfacefictive temperature measured to be 711° C., a surface compression of−201 MPa, a central tension of 67 MPa, and a Vicker's cracking thresholdof 20-30 N.

Example 13—A CORNING® GORILLA® Glass plate of 1.1 mm thickness ispre-heated for 10 minutes at 550° C. before transferring to the hot zonewhere it is held at a T₀ of 800° C. for 10 seconds. After equilibratingit is rapidly transferred to the quench zone, which has a gap of 91 μm,where it is held for 10 seconds. The resulting article has a surfacefictive temperature measured to be 747° C., a surface compression of−138 MPa, a central tension of 53 MPa, a flatness of 66 μm, and aVicker's cracking threshold of 20-30 N.

TABLE VI Thickness Gaps CS CT Flatmaster Fictive Vickers Example (mm)Composition (um) T₀ Gas (MPa) (MPa) (um) (° C.) (N)  1 5.7 SLG 91 690Helium −312 127 83 — —  2 5.7 SLG 91 690 Helium −317 133 90 — —  3 1.1SLG 56 700 Helium −176 89 190 661.3 10-20  4 0.55 SLG 25 720 Helium −17663 125 — —  5 1.5 GG 226 790 Helium — — 113 before/ — — 58 after  6 0.7SLG 31 730 Helium −206 100 82 — —  7 3.3 Borofloat 33 119 800 Helium — —121 — —  8 3.2 SLG 84 690 Helium −218 105 81 — —  9 0.3 SLG 159 630Helium — — — — — 10 0.1 GG 141 820 Helium — — — — — 11 1.1 SLG 65 700Helium −201 98 158 657 10-20 12 1.1 GG 86 810 Helium −201 67 — 711 20-3013 1.1 GG 91 800 Helium −138 53 66 747 20-30

Additional Example—a 5.7 mm thick sheet of glass comprising at least 70%silicon dioxide by weight, and/or at least 10% sodium oxide by weight,and/or at least 7% calcium oxide by weight was run with helium gas andgaps 204 a, 204 b (FIG. 21 ) of about 90 micrometers. The glass washeated to an initial temperature of about 690° C. and quickly cooled.The resulting strengthened article had a negative tensile stress ofabout 300 MPa on surfaces thereof and a positive tensile stress of about121 MPa in the center. Also, the resulting strengthened article had aflatness of about 106.9 micrometers.

Additional Example—In one experiment using inventive technologydisclosed herein, a 1.1 mm thick sheet of glass comprising at least 70%silicon dioxide by weight, and/or at least 10% sodium oxide by weight,and/or at least 7% calcium oxide by weight was run with helium gas andgaps 204 a, 204 b (FIG. 21 ) of about 160 micrometers. The glass washeated to an initial temperature of about 680° C. and quickly cooled.The resulting strengthened article had a negative tensile stress ofabout 112 MPa on surfaces thereof and a positive tensile stress of about54 MPa in the center. Prior to strengthening, the sheet of glass had aflatness of about 96 micrometers, but the resulting strengthened articlehad a flatness of about 60 micrometers. Accordingly, the strengtheningprocess also flattened the strengthened glass or glass ceramic article.

Other aspects and advantages will be apparent from a review of thespecification as a whole and the appended claims.

The construction and arrangements of the cover glass and glass-ceramic,as shown in the various exemplary embodiments, are illustrative only.Although only a few embodiments have been described in detail in thisdisclosure, many modifications are possible (e.g., variations in sizes,dimensions, structures, shapes, and proportions of the various elements,values of parameters, mounting arrangements, use of materials, colors,orientations) without materially departing from the novel teachings andadvantages of the subject matter described herein. Some elements shownas integrally formed may be constructed of multiple parts or elements,the position of elements may be reversed or otherwise varied, and thenature or number of discrete elements or positions may be altered orvaried. The order or sequence of any process, logical algorithm, ormethod steps may be varied or re-sequenced according to alternativeembodiments. Other substitutions, modifications, changes and omissionsmay also be made in the design, operating conditions and arrangement ofthe various exemplary embodiments without departing from the scope ofthe present inventive technology.

What is claimed is:
 1. A glass sheet for use in or on consumerelectronic products, the glass sheet comprising: glass comprisingsoda-lime glass, alkali aluminosilicate glass, alkali-containingborosilicate glass, alkali aluminophosphosilicate glass, or alkalialuminoborosilicate glass; a first major surface opposite a second majorsurface with an interior region located therebetween; wherein surfacefictive temperature measured on the first major surface is at least 50°C. above a glass transition temperature of the glass of the glass sheet;wherein an average thickness between the first and second major surfacesis less than 2 mm; wherein an ion content and chemical constituency ofat least a portion of both the first major surface and the second majorsurface is the same as an ion content and chemical constituency of atleast a portion of the interior region; wherein the first and secondmajor surfaces are under compressive stress greater than 150 MPa and atleast a portion of the interior region is under tensile stress; whereina surface roughness of the first major surface is between 0.2 and 1.5 nmRa roughness.
 2. The glass sheet of claim 1, wherein stress within theglass sheet varies as a function of position relative to the first andsecond major surfaces, wherein the stress has a change of at least 200MPa over a distance of less than 500 μm of thickness of the glass sheet.3. The glass sheet of claim 1, wherein a surface roughness of the secondmajor surface is between 0.2 and 1.5 nm Ra roughness.
 4. The glass sheetof claim 1, wherein the first and second major surfaces are flat to atleast 50 μm total indicator run-out along a 50 mm profile of the firstand second major surfaces.
 5. The glass sheet of claim 1, wherein theareas of the first and second major surfaces are at least 2500 mm². 6.The glass sheet of claim 1, wherein thickness of the glass sheet isabout 0.1 mm to about 1.5 mm.
 7. The glass sheet of claim 6, wherein theglass sheet at the thickness thereof exhibits an average lighttransmission over a wavelength range from 400 nm to 780 nm of 85% orgreater.
 8. The glass sheet of claim 6, wherein the glass sheet furthercomprises a depth of compression that is greater than 17% of thethickness.
 9. The glass sheet of claim 1, wherein the glass of the glasssheet has a coefficient of thermal expansion greater than 40×10⁻⁷/° C.10. A glass sheet for use in or on consumer electronic products, theglass sheet comprising: glass comprising soda-lime glass, alkalialuminosilicate glass, alkali-containing borosilicate glass, alkalialuminophosphosilicate glass, or alkali aluminoborosilicate glass; afirst major surface opposite a second major surface with an interiorregion located therebetween; wherein surface fictive temperaturemeasured on the first major surface is at least 50° C. above a glasstransition temperature of the glass of the glass sheet; wherein anaverage thickness between the first and second major surfaces is lessthan 2 mm; wherein an ion content and chemical constituency of at leasta portion of both the first major surf ace and the second major surfaceis the same as an ion content and chemical constituency of at least aportion of the interior region; wherein the first and second majorsurfaces are under compressive stress greater than 150 MPa and at leasta portion of the interior region is under tensile stress; wherein asurface roughness of the first major surface is between 0.2 and 1.5 nmRa roughness, and wherein a surface roughness of the second majorsurface is between 0.2 and 1.5 nm Ra roughness; wherein stress withinthe glass sheet varies as a function of position relative to the firstand second major surfaces, wherein the stress has a change of at least200 MPa over a distance of less than 500 μm of thickness of the glasssheet.
 11. The glass sheet of claim 10, wherein the first and secondmajor surfaces are flat to at least 50 μm total indicator run-out alonga 50 mm profile of the first and second major surfaces.
 12. The glasssheet of claim 10, wherein the areas of the first and second majorsurfaces are at least 2500 mm².
 13. The glass sheet of claim 10, whereinthickness of the glass sheet is about 0.1 mm to about 1.5 mm.
 14. Theglass sheet of claim 13, wherein the glass sheet at the thicknessthereof exhibits an average light transmission over a wavelength rangefrom 400 nm to 780 nm of 85% or greater.
 15. The glass sheet of claim13, wherein the glass sheet further comprises a depth of compressionthat is greater than 17% of the thickness.
 16. The glass sheet of claim10, wherein the glass of the glass sheet has a coefficient of thermalexpansion greater than 40×10⁻⁷° C.
 17. A glass sheet for use in or onconsumer electronic products, the glass sheet comprising: glasscomprising soda-lime glass, alkali aluminosilicate glass,alkali-containing borosilicate glass, alkali aluminophosphosilicateglass, or alkali aluminoborosilicate glass; a first major surfaceopposite a second major surface with an interior region locatedtherebetween; wherein surface fictive temperature measured on the firstmajor surface is at least 50° C. above a glass transition temperature ofthe glass of the glass sheet; wherein an average thickness between thefirst and second major surfaces is less than 2 mm; wherein an ioncontent and chemical constituency of at least a portion of both thefirst major surf ace and the second major surface is the same as an ioncontent and chemical constituency of at least a portion of the interiorregion; wherein the first and second major surfaces are undercompressive stress greater than 150 MPa and at least a portion of theinterior region is under tensile stress; wherein a surface roughness ofthe first major surface is between 0.2 and 1.5 nm Ra roughness; whereinthe areas of the first and second major surfaces are at least 2500 mm²;and wherein the glass sheet further comprises a depth of compressionthat is greater than 17% of the thickness.
 18. The glass sheet of claim17, wherein stress within the glass sheet varies as a function ofposition relative to the first and second major surfaces, wherein thestress has a change of at least 200 MPa over a distance of less than 500μm of thickness of the glass sheet.
 19. The glass sheet of claim 17,wherein a surface roughness of the second major surface is between 0.2and 1.5 nm Ra roughness.
 20. The glass sheet of claim 17, wherein thefirst and second major surfaces are flat to at least 50 μm totalindicator run-out along a 50 mm profile of the first and second majorsurfaces.